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01 fluids lead photo

What Fluids Are In Your Vehicle?

Everything You Need To Keep Your Cool

Photography by Harry Wagner

They say that the human body is 60% fluid. Your 4×4 doesn’t have that much fluid in it, but the host of fluids are still vital to ensure that your vehicle functions properly. While we don’t expect you to carry a full oil change with you on the trail, spare fluids are worthwhile to ensure you make it home when you venture out into the backcountry. Additionally, maintenance schedules for things like engine oil will be accelerated with heavy off-road use in dusty environments, and if you regularly cross water you will want to check your differentials for water contamination and grease u-joints more often than vehicles that stick to the pavement. The goal of the fluids you carry with you on the trail is just to get you home safely. Once you are back to civilization more maintenance may be required to get your vehicle back to tip top condition before your next adventure.

02 spare fuel cans


Fuel is likely the fluid you check most often in your vehicle, regardless of whether it uses gasoline or diesel. Larger replacement fuel tanks and auxiliary tanks are available from companies like Titan Tanks that provide increased range between fill ups. Alternatively, spare fuel can be carried in containers such as those available from Wavian and Rotopax and fitted to bed racks in pickups or rear bumpers on Jeeps. Never carry spare fuel inside your vehicle, as the containers can vent or spill during temperature and altitude changes.

03 trail side oil change

It is better in many instances to be half a quart low than half a quart overfilled, as overfilling can cause the oil to hit the crank and aerate. Aeration is when air bubbles get into the fluid and reduce its ability to lubricate, which is a problem for any fluid from engine oil to power steering fluid.

Engine Oil

Oil is vital to keeping your engine lubricated and running cool. If you are performing regular maintenance you shouldn’t need to add any on the trail, in most vehicles the “full” to the “add” marks on the dipstick represent a quart of oil. We carry a quart of oil with us on the trail to top off the fluid if it is too low, which can lead to low oil pressure and accelerated wear. We don’t carry enough oil for a full oil change due to space concerns, but we have had to do an oil change on the side of the trail before after a particularly deep river crossing filled the oil pan with water. Ideally you want to use the same brand and viscosity of oil in your engine, but in this case we had to borrow a quart of oil from multiple people in order to fill the crankcase. The result was still a far better source of lubrication than the water was. Also make certain you have containers with enough space to carry the contaminated fluid out with you until you can properly dispose of it.

04 adding distilled water to radiator

Back 25 years ago, antifreeze was green, you mixed it 50-50 with water and you called it a day. Today, times have changed, with antifreeze every color of the rainbow and specific to European, Japanese, and specific domestic manufacturers.


Older vehicles (pre-1990) typically all came with green IAT (inorganic acid technology) coolant while newer vehicles use OAT (organic acid technology) that comes in a variety of colors. The newer OAT coolant is formulated to work with aluminum components and silicone seals better than the IAT of old. It isn’t a good idea to mix the two types of coolants, but both are mixed with water. For this reason we typically don’t carry dedicated coolant with us on the trail, instead we just use drinking water when we have to repair a radiator or replace a blown hose. Distilled water is the best choice since it doesn’t have the corrosion-causing minerals and silicates that tap water has. Long term you don’t want to run only water in the radiator as it lacks the lubrication of coolant and will not prevent freezing, which can crack your radiator or even your engine.

05 checking automatic transmission fluid

Fluid in an automatic transmission should be checked with the vehicle warmed up and the engine running with the transmission in neutral.  Manual transmissions are more of a challenge to check, they typically have two plugs on the side of the case, a lower one to drain the fluid and an upper plug to fill the transmission.  Pull the fill plug and stick your finger in the hole to check the fluid level.

Transmission Fluid

Manual transmissions typically use gear oil, although some such as the M50D and the ZF5 actually use ATF. Note that even in the appropriate weight (such as 75W90), there are distinctions for GL4 and GL5 with gear oil used in manual transmissions. The main difference between GL-4 and GL-5 gear oils is the amount of additives. There is twice as much sulphur and phosphorus in GL5 compared to GL4, and it can damage bronze and copper synchros. Save the GL5 for your differentials unless your transmission specifically calls for it.

Automatic transmissions are even more complex. While you could technically rockcrawl through a trail with a manual transmission that was completely devoid of fluid (we don’t recommend it though), this would be impossible with an automatic. The fluid is a critical part of a torque converter to provide coupling and forward motion. ATF comes in over a dozen different varieties these days, but if you need to get off the trail and you only have Dextron to put in your Ford C6, it will get you home. Just be certain to drain and replace the filter and appropriate fluid when you have the opportunity, as different ATF designations have specific additives specific to different manufacturers. We carry a quart of the appropriate fluid with us on the trail, which is enough to top off the transmission but completely refill it should the pan get punctured.

06 contaminated gear oil

If you like to play in the water, check your gear oil frequently.  Water provides little to no lubrication and can cause failure of bearings and gear sets.  It will also rust carriers and ring and pinions if left to sit for too long.  Running raised differential breathers is a good way to prevent water from entering your differentials.

Gear Oil

Your differentials (and typically your transfer case) use gear oil to stay cool and lubricate the moving parts. The most common gear oils are 75W90 and 75W140, with the 140 being thicker. As a result it does not flow as well in cold conditions and can suck up some horsepower, but being thicker it also does a better job of cushioning against shock loads that are common when wheeling and the heat generated when towing. Leaking gear oil is typically difficult to miss due to the smell and the drips it leaves. We carry a quart of gear oil with us on the trail in case we put a hole in a differential cover or roll over and some fluid comes out of the breather. If you have a limited slip differential that uses clutches, note that a friction modifier is necessary to add to the gear oil in order to keep the clutches from chattering and wearing out prematurely. We don’t carry the additive with us on the trail though as it isn’t critical to making it home under your own power.

07 swepco power steering fluid

PSC requires Swepco power steering fluid for all of their boxes and pumps if you want them covered under warranty, citing that this fluid resists the foaming that can prematurely kill steering components.  This fluid typically comes with the parts when you buy them new and can be sourced from 4Wheel Parts when you need to replace it.

Power Steering Fluid

Power steering fluid is another hydraulic oil, similar to ATF. ATF, however, contains friction modifiers and detergents to clean out the automatic transmission’s dirt and grease away yet damage the hydraulic valves of the steering rack and pump. If your steering is leaking or low on the trail though, ATF will do the trick. For this reason, we don’t carry dedicated power steering fluid with us on the trail.

08 brake fluid reservoir

Brake fluid is hygroscopic, which means it absorbs water, which can lead to rust, corrosion, and a lower boiling point for brake fluid. Brake fluid is one of the fluids you can change from time to time, because it does go bad as it absorbs water. 

Brake Fluid

We carry a 12-ounce spare brake fluid container on the trail in case we need to bleed the brakes after damaging a brake line.  Typically, once we have opened the fluid we try to use it all.  There isn’t much value in keeping an open container of brake fluid because it will absorb water and become less effective, that is why we buy smaller containers. Brake fluid is typically listed as DOT3, DOT4, and DOT5.  DOT4 has a higher boiling point than DOT3, which is a good thing.  It is also more prone to absorbing water than DOT3 fluid though, which negates many of the benefits.  DOT5 fluid is silicone based and not compatible with DOT3 and DOT4 fluid, so we generally avoid it in our off-road vehicles. We have mixed DOT3 and DOT4 fluid without issue though.

09 synthetic fluid

In the past you may have heard that switching from conventional to synthetic oil would result in issues like burning oil or leaking seals. At this point, these theories have all but been disproven though.  There is no risk in switching back and forth between conventional and synthetic fluids.

Synthetic or Conventional?

Nearly all of the fluids listed above are available in both traditional and synthetic form. Synthetic oils tend to cost more than conventional oils, but they have a high purity and consistency since they are designed that way in a laboratory. That means they provide better longevity, which allow them to pay for themselves with increased service intervals. If you find yourself changing your fluids often though due to water contamination, heavy use, or high heat, you negate the cost saving of longer service intervals. There are still advantages to synthetics though in the resistance to sheering and improved lubrication they can provide.

Products we used in this article