Off-road driving is dirty, tough work. That’s why you need to select the right oil and change it at the right times. Those are two important steps to keeping your rig happy and healthy. Our focus here is on choosing the right oil for the way you drive.
Reading oil specs
The packaging of the oil will describe the product’s basic characteristics. For more specific information, you can go to the Internet as a starting point. Try the oil companies’ websites, as that’s where you can find lots of data. Look specifically for the Material Data Sheets, also known as MDS. Comparing MDS among oil brands may be difficult, as they don’t always show the same information. If you want a specific number or if you have a specific question, you can always call or email the oil company’s tech hotline. Another resource is BITOG. This is an online community of experts, chemists, and drivers who communicate and share knowledge.
Before you jump into the extra research though, you should make sure you know what the specs mean. Oil specs are usually modeled after the procedures established by American Society for Testing and Materials (ASTM). Some data are more telling, but the most telling is how different oils compare on the same tests.
You will see this shown as SUS (100F and 210F) and cSt (at 40C and 100C).
Viscosity index (VI)
This number tells you how the oil’s thickness changes as the temperature changes. When the VI number is high, it means the oil has a synthetic base or it has additives called viscosity index improvers (VII). Higher viscosity index is better, and both the synthetic oils and the oils with additives will perform well initially. The oil with the additives, however, will break down faster than the synthetic. Also, some VIIs are good and others are not as good. This means a higher quality oil should last longer than a lower quality one, even if both have the same VI. As general rules of thumb, synthetic oils have a higher VI than mineral oils, and multi-grade oils compare better than single-grade oils.
Flash point (FP)
The FP is the temperature at which the oil will vaporize. When a flammable liquid vaporizes, it forms an ignitable gas that can burn, and keep burning, just above the surface of the liquid. The FP measurement is obtained by slowly heating the oil and then recording the lowest temperature at which the ignition occurs. A higher FP is better. An oil with a low FP vaporizes under the high-temperature conditions of an engine. Therefore, it will be used faster than an oil with a higher FP.
About 400ºF is the lowest acceptable FP for a new oil. It’s better to shoot for 420ºF for mineral oils and 450ºF for synthetics. One point to note is that fuel dilution lowers an oil’s FP.
This measure indicates how quickly the oil will evaporate due to high temperatures. The lower the NOACK volatility number is, the better; this means the oil has a low potential for burning off.
Total base number (TBN)
TBN indicates how resistant an oil is to acids and contaminants over time. A higher TBN is better, but it’s more important to know how quickly the number goes down over time. In higher quality oils, the TBN may drop slowly, while lower quality oils may show a quick drop in TBN. This data is only available through an oil analysis. Oils rated for gas have a minimum TBN of about 6. Dangerously low would be 2. Oils rated for diesel have a higher TBN, around 10, because they contain more detergents.
High temperature high shear (HTHS)
This is a less common metric for viscosity, but it tells you how well an oil keeps its protective properties under severe conditions. HTHS, which is rated at a specific temperature, will differ by SAE grade. Generally, the higher the viscosity, the higher the HTHS number will be. However, when the number is higher than the minimum for any viscosity, it indicates the oil is more shear-resistant.
Mineral oils tend to test lower than synthetics of the same grade. With SAE 30 and 40 gas engine oils, the lowest HTHS is roughly 2.9 cP. The minimum is about 3.7 cP for diesel engine oils with SAE of 40 or higher. Oils with HTHS above 4 cP are strong. Within a particular grade, a higher HTHS is better.
Pour point is a temperature rating, indicating how warm the oil needs to be before it can be poured. At any temperature below the pour point, the oil will be semi-solid. The pour point itself, however, is not a usable temperature. At this temperature, the oil has the texture of honey. This metric is more telling with respect to gear oils. You can reduce an oil’s pour point by adding pour point improvers.
Cold cranking stimulator (CCS)
This measurement might also be called “low temperature pumping viscosity,” and it helps you evaluate how an oil performs in cold temperatures. CCS is applicable to the winter grade part of the oil. Different grades have varying minimum ratings, taken at various temperatures:
0W rated oils are measured at -35ºC
5W rated oils are measured at -30ºC
10W rated oils are measured at -25ºC
15W rated oils are measured at -20ºC
20W rated oils are measured at -15ºC
If you compare two numbers at the same temperature, the higher number will mean a poorer cold flow.
4-Ball wear test/4-ball EP test
Gear oil specs might include the 4-ball wear test or 4-ball EP test metrics. In the first test, one ball bearing is spinning and rubbing against the other three ball bearings in the test lubricant. The test itself measures the scars or marks on the ball bearings under test loads. The EP test is a more common number, used to rate gear oils. It quantifies how much force is needed (in kilograms per square inch) to weld the bearings together. A higher number is better.
What the brand name means
All motor oil products on the market fall into one of three groups:
The big boys like Chevron, Shell, and Texaco
The boutique oil makers including Amsoil, Lubrication Engineers, Royal Purple, Redline, and Schaeffers
Private brand blenders that bottle for Wal-Mart, Meijer, and Tractor Supply.
Oils made by any of these companies should provide acceptable performance. However, the adage that “you get what you pay for” holds true. The manufacturers provide oils at all different price points. Unless you stumble on a close-out sale, you won’t find a top base oil and additive package on the cheap. You might save a few bucks, though, by shopping around.
You can count on the big brands to provide a reasonable value in any oil category. The Chevrons and Shells have the advantage of scale — their size allows them the ability to produce a good value for the price. They can also fund productive R&D departments, and these folks are responsible for much of the progress and advancements made in oil technology.
The boutique makers have smaller production footprints, but their formulations are often excellent. They strive to combine the best base oils with the best additives, producing the fine wines of oil. The drawback is the pricing, which is higher than what you might pay for a Chevron or Shell formulation.
The last group is hit or miss. You don’t see much of these guys in the marketplace. They focus on private brand contracts for auto parts stores, grocery stores, department stores, and dollar stores. These oils can be good or bad, depending on what the contract requires.
Choosing an acceptable oil
Anything that’s a step-up from the no-name oils will likely provide acceptable performance. A less expensive oil will need to be changed more often, which offsets the lower price per bottle. Changing the oil too often is also inefficient for other reasons; sometimes you end up dumping good oil out with the bad. You aren’t doing yourself or your engine any favors by changing the oil more often than necessary. A higher quality oil saves you money because it lasts longer.
When you make your choice, you should base it on these four standards: manufacturer’s requirements, driving conditions, service interval, and climate. If there is a conflict among these standards, let the most common factor guide your choice.
Most car and truck manufacturers do not have specific motor oil requirements. They may encourage you to use a certain type, but it’s usually just a suggestion rather than a requirement. Your car or truck maker may have some guidelines concerning viscosity and API service rating.
Any special requirements or guidelines should be considered. You might see these special requirements for manual transmission oil, power steering fluid, and automatic transmission fluid. Boutique blends for these fluids may be superior to the mass market products available.
You can always read the bottle also, to see if the lubricant is appropriate for your vehicle. You may see verbiage like “Suitable for.” Do not confuse this with the more preferable statement, “Meets or exceeds.” Also, avoid making any drastic departures from the manufacturer’s recommendations, as this could impact your warranty.
Your driving conditions should dictate the viscosity of your oil. You want an oil with higher viscosity when you drive in very warm temperatures or if you tow or haul heavy loads. The same is true if your engine is mostly driven at lower rpm, or if the oil temperature is consistently around 212 degrees.
This decision should also consider potential cold climate conditions. A lower viscosity oil is more appropriate for colder climates.
These rules apply to axle and gear oil. A truck that is driven in hot weather and/or tows or hauls frequently would benefit from the use of higher viscosity lubricants.
Your manufacturer should specify the service interval requirements in your owner’s manual. The service interval should be influenced by the oil you’re using and your driving cycle. Ideally, you should drive in cycles long enough to heat up the oil to full operating temperature for extended timeframes. Freeway driving is an example. A mineral that is moderate-quality or better should last 7,500 miles with a decent driving cycle.
Shorter driving cycles are more draining on the oil’s useful life. When oil heats, it sheds contaminants and burns off fuel dilution burns. In short driving cycles, the oil never reaches the temperatures necessary to complete this self-cleaning process. If you do drive only short distances, you should change the oil in half as many miles. Going beyond 7,500 miles with a mineral isn’t a good idea, unless you have oil analysis data to guide the decision. Good synthetics outperform the minerals with longer intervals in all driving cycles, but how far you go past 7,500 miles depends on other factors. One of these factors is the quality of your oil filtration.
Time can be another factor. Once oil is exposed to the air, it can begin to break down — even if the vehicle is not being used. Better oils make use of oxidation inhibitor additives to slow this process. These oils can be used safely for up to a year, assuming the conditions are right. The right conditions include a long driving cycle and low humidity. If your vehicle is exposed to humidity and is driven only short distances at a time, it’s safer to change the oil every three or six months.
When temperatures rise, viscosity improves. Likewise, when temperatures fall, viscosity declines. This is true for engine oils and gear oils, but it may not apply to ATF or manual light transmission oils.
In very hot climates, your truck might do best with an SAE 40 oil. Take that same truck and driving cycle to a cooler climate and an SAE 30 may be more appropriate. Head to Alaska, and then the right choice might be SAE 20. Again, check your owner’s manual for specific requirements. You’ll get the most climate flexibility from the winter part of a multi-grade oil.
Mineral, synthetic, and blended
Today, the word “synthetic” doesn’t carry the weight it used to. Mineral oils have been improved to the point that “synthetic” signals a compositional difference much more than a performance difference. In other words, you don’t need to limit your choices to synthetics. Instead, evaluate an oil’s performance relative to how you drive, then consider the price. Often, a high-quality mineral oil will be more than good enough for what you need, and so you don’t need to spend more on a fancy synthetic. One more point to note is that “synthetic” could describe one of the following:
a Group III mineral base
a Group IV PAO
a Group V ester synthetic base
Blended oil will combine synthetic into a Group II or higher mineral base. The amount of synthetic used can be as low as 10%. The result is better performance, but it comes at a price. A blend with 10% synthetic could be more than 10% higher in price than a full mineral oil. In this case, you are paying more for the upgraded performance than it’s worth.
You should pay attention to how the base oil performs at extreme temperatures. An extreme climate, for example, could push you to pick a synthetic, particularly if you live in very cold temperatures. A synthetic oil also makes sense if you want to stretch out your drain intervals. Also, a synthetic (PAO or ester base in particular) has a better lubrication quality, so you could squeeze out a tiny bit more power from a synthetic over a mineral-based oil.
Synthetics are most useful for axles. The conditions under which your axles operate are extreme. The temperatures are high, there’s no oil filter, and the shock loads and hypoid action force the oil to perform. These strains are even greater for off-road vehicles because of the heated stresses of towing and higher shock loads associated with driving on uneven terrain. A quality synthetic oil will reduce friction and that will keep axle oil temperatures lower. This temperature reduction, relative to a mineral oil, could be 30-35 degrees or more. Also, a heavier synthetic can be used, because these formulations have strong cold flow qualities. An SAE 110 and SAE 140 will generally work fine, even when it’s cold. The higher viscosity of these, relative to an SAE 90, provides improved shock load protection in a 4x4 vehicle. The drawback is that if the axle isn’t really hot, the heavier oil creates more friction, which can impact fuel economy. Note that a synthetic SAE 140 has better friction performance than a mineral SAE 140, which means less parasitical drag.
Even the best engine oil needs to be filtered to remain functional. Potential contaminants from the inside of the engine include wear metals and carbon from combustion. Dust from the outside can also get in through the air filter. The dust might stick on the cylinder walls temporarily, before being swept away by splashes of oil. These may be very small particles, but they need to be removed before they cause any damage.
The most common particles are 10-20 microns in size. These are also the most destructive. The industry standard is to provide 10-micron filtration, but there are premium filters that collect even smaller contaminants. These smaller particles, those in the 5-10 micron range, cause less immediate damage, but they do contribute to long-term wear.
Internal engine clearances are 30-40 microns, which means that particles of that size can cause wear. This is even more of an issue in those times when the hydrodynamic layer lessens or fails completely.
Today’s lubrication systems filter all of the oil, using full-flow filters. However, oil filtration depends on how fine the filter is, how quickly the oil flows, and the filter’s holding capacity. A finer filtration reduces the flow, but too low of a flow can lead to engine damage. You can make adjustments for this conflict by using more filtering media, but you are limited by space. More effective filter media can remove very small particles, but it also is more likely to clog, which also reduces flow.
Advances in synthetic media address some of these problems by providing better filtration without impacting flow and capacity. Many synthetic filter products perform better than the traditional pressed cellulose media. Another option is a bypass filter kit, which takes about 10 percent of the oil at a time and filters it very finely. These kits can catch particles as small as 1 micron.
You can distinguish a good filter from a not-so-good filter by the specs. High quality filters generally provide their specs, but even the other guys should provide the specs if you request them. Some of the common specs you’ll see include the following.
Nominal micron rating
This number is the particle size, in microns, of which the filter can effectively collect 50%. Said another way, the filter will pull out half of the particles that are of the size stated.
Absolute micron rating
The absolute micron rating is the more useful figure. More than 98% of the particles of this size will be collected by the filter.
This ratio is relatively complex. It reflects the percentage of particles that get through the filter under various conditions, including pressure, temperature, and fluid viscosity. The percentage is determined by counting the number and size of particles that aren’t captured by the filter in the various scenarios.
For example, a K&N 6001 filter lists these data for its beta ratio: 10/20/30=2.1/21.7/1000. The 10, 20, and 30 are the sizes of the particles tested, in microns. The numbers after the equals sign are the beta ratios. To deduce the percentages from the beta number, you have to perform a few calculations:
Subtract 1 from the beta number, as in 2.1-1=1.1
Divide the difference by the beta number, as in 1.1/2.1=0.523
Multiply the result by 100, as in 0.523x100=52.3%
Rounding accounts for the difference between our 52.3% and K&N’s published 53.1%. You can use this table of beta ratios and their related efficiencies as a reference going forward:
2 beta = 50% efficiency
10 beta = 90% efficiency
20 beta = 95% efficiency
75 beta = 98.7% efficiency
100 beta = 99% efficiency
200 beta = 99.5% efficiency
1,000 beta = 99.9% efficiency
The flow rate represents how well the filter, when it’s new, allows the oil to flow. Flow rates go down as the filter captures more particles. This metric is usually presented in gallons per minute (GPM) or liters per minute (LPM).
Capacity indicates how much material the filter can collect, in grams, before the pressure drops by 8 psi.
An anti-drainback value keeps the oil in the filter from being siphoned out or from draining. Some, but not all engines require these. The best are those made of silicone, which are usually red in color. Anti-drainback valves made of nitrile are common in less expensive filters. Nitrile is not as durable as silicone and can harden with too much heat exposure.
Some filters have a bypass valve that allows unfiltered oil to move past a blocked filter. These valves are necessary in engines that do not have an internal bypass valve. You can find filters with flapper style bypass valves or coil spring-backed valves; the latter are preferable.
Canister and endplate
The durability of the filter canister is important, in the event a rock or some other piece of debris makes contact with your filter. Racecar drivers are more typically concerned with burst pressure, but you can interpret a higher burst pressure number to mean a stronger filter. You might also deduct durability by the filter’s endplate design. The size and number of holes impact flow rate.
You might be lucky enough to find the filter area specified in square inches. If not, you can autopsy the filter to find out. When you do, you may find that a filter with a large canister doesn’t have an equally large filter element. Bigger elements are better.
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