Dennis Kirk Inc.

Motorcycle Fluids Guide

Keeping your ride in tip-top shape

The proper fluids are necessary for our motorcycles to operate correctly. But how do you make sure that you choose the right ones? Should you buy standard or synthetic and what weight do you need? Can you just use automotive fluids? These are some of the questions that get asked frequently by many riders. If you are servicing your bike's fluids, it's important to know how to choose the ones that will work best in your machine. Follow this guide to learn more about each type of motorcycle fluid and how to pick which ones are right for you.

motorcycle oil


Motorcycle oil serves three main purposes within an engine and transmission. The first is to provide lubrication for all of the moving parts. A thin film of oil coats these parts to ensure that metal parts do not wear on other metal parts. The second main purpose of oil is to aid in cooling. As the oil moves through the engine, it helps to transfer the heat to the outer portions of the engine where it can then dissipate to the outside of an air cooled engine or to come into contact with the cooling system on a liquid cooled. Lastly, engine oil acts to help keep the inside of the engine clean. Oil will hold carbon and other particles in suspension and will also help to prevent sludge and debris from collecting on internal engine parts.

At first glance, it may seem that all engine oil is the same and that you should even be able to use regular automotive oil in your motorcycle, right? The answer is no, not quite. While all petroleum, synthetic and blended oils start off with roughly the same "base" oil respectively, the additive packages are what differentiates a motorcycle oil from a car oil. The additive package in a motorcycle oil is specifically designed to handle the added stresses of a motorcycle engine along with working with wet clutches.

Additives in engine oils are used to help the oil perform a variety of duties while in the engine, aside from lubrication and cooling. Detergents and dispersants are a very common additive in all oils to help keep the inside of the engine clean by holding debris and not allowing it to stick to anything. To aid in keeping the engine clean, there are also solvents added to break up any deposits that do form. Other common additives include Viscosity Index improvers, and corrosion inhibitors and buffers, which help to neutralize acids.

There is one more additive, but it should never be found in your motorcycle oil. Regular automotive oils use an additive called molybdenum to act as an emergency lubricant in case the base oil fails. The problem with molybdenum in motorcycles is that it can make the clutch slip on any wet clutch bike. Motorcycle oils do not have molybdenum as an additive and any oil that does have it as one should be avoided in your motorcycle. Instead of molybdenum, motorcycle oils primarily rely on phosphorus and zinc as emergency lubricants as they will not cause the clutch to slip. Motorcycle oil will also have added friction modifiers so that it will not shear as easily from being ran through both the engine and the transmission.

Synthetic vs. mineral based motorcycle oil is a question that almost always stirs up a heated debate when asked among seasoned riders. The answer to the question? It depends. For the most part, a synthetic oil is the best choice, but there are times when a mineral based oil will outshine a synthetic.

With the advances in oil technology, oil companies have been able to develop additive packages that will make a mineral based oil perform better than a lower quality synthetic. With motorcycle oils, the old saying, "you get what you pay for" really rings true. The best synthetics may cost a bit more, but they will almost always outperform a mineral based oil.

When deciding on what weight oil to choose, it's important to consult your owner's manual. Using the manufacturer's recommendation is always the safest. Multi-grade oils will keep the engine protected in a much wider range of temperatures compared to a single grade oil. Cold starts are less of a problem and the engine is better protected if it happens to rise above normal operating temperatures with a multi-grade oil.

Motorcycle oil should be changed more regularly than an automobile because they are harder on the oil. Oil in a motorcycle is often used in both the engine and the transmission, plus it is ran at higher RPM than an automobile. You should follow the guidelines of your service manual on how often to change the oil. If you do not have a manual, 1500 miles is a safe number to shoot for in between changes.


For long lasting cooling performance, you should only use motorcycle and powersport specific engine coolant/antifreeze. There are two kinds of coolant; propylene glycol and ethylene glycol. Propylene glycol is often accepted as the best option for motorcycles. The two types of coolant should never be mixed. If you are unsure of what type of coolant is in your system, make sure that you never mix different colors of coolant together. The different types are dyed separate colors to help users better identify them.

You should also look for coolant that does not contain any silicates or phosphates. Silicates and phosphates in coolant will form a layer over the metals of the coolant system to help insulate it. If left unmaintained, the coolant will break down as the silicates and phosphates create the layer and are removed from the liquid. The layer can also become too thick and not allow enough heat to be transferred from the engine into the coolant, causing the engine to overheat. Another problem with silicates and phosphates is that they can "eat" water pump seals. The coolant that you use should also be safe to use with aluminum as many motorcycle radiators are made with it.

Motorcycle coolant should be changed fairly regularly. One year is often a good start, but it can be stretched out longer if needed. The coolant should be mixed with ionized water to help prevent scale buildup that can be caused with impure water.

Another plus side of propylene glycol is that it is actually not poisonous if it happens to be ingested. This makes it much more safe to use around pets and children.

Brake Fluid

Being able to stop is equally important to being able to go on a motorcycle. You need your braking system to be reliable and consistent. One way to ensure that your bike will stop when you need it to is to have the proper brake fluid that is also fresh. You should change your motorcycle brake fluid every 1-2 years with the proper fluid.

Most brake fluid is Glycol based and is hygroscopic. That means that it will absorb moisture when exposed to it, even if it is just in the air. Although your brake system is in theory a "sealed system," moisture can still make its way through seals and hoses. As time goes by, the fluid will continue to absorb moisture, which will water down the fluid and degrade its performance capabilities. The water also lowers the boiling point of the fluid, which makes even less effective once the bike warms up. When refilling your brake fluid, you should always try to use a new bottle that was sealed. Once the seal is broken, the fluid will begin to absorb moisture even with the cap on.

You should always follow the manufacturer recommendation for the brake fluid rating. If you cannot find it, know that motorcycles should be filled with either DOT 4 or DOT 5.1 (NOT DOT 5, which is silicone based, unless it is specified) brake fluid. Automotives generally will run DOT 3. The DOT 4 and DOT 5.1 brake fluids have a higher heat rating, which is required in a motorcycle's system. The lower rating has a higher likelihood of boiling in the system.

Transmission Oil

For some motorcycles, you won't have to worry about transmission/gear oil. A "wet clutch gearbox" shares the engine oil for lubrication. Other motorcycles have a gearbox that is independent of the engine. Motorcycles with the independent gear boxes will almost always require an oil with a higher viscosity. The reason for this is that these bikes need an oil with a higher shear tolerance so the oil will not immediately break down while in use. Transmission oils often have additives to increase the shear strength and to help prevent slippage. You should always try to use the recommended weights and oils for your particular machine.

Fork Oil

Motorcycle fork oil should be changed according to the manufacturer specifications, which is usually every 1-2 years. Bikes that are ridden hard or off road may need to be changed earlier. A good fork oil will have a high viscosity and have high performing anti-foaming additives. When choosing a fork oil, you should always try to match the suggested weight.

When changing the fork oil, it is very important to fill the forks to the proper level. Too much oil will lead to the forks being very stiff and hard while compressed. Too little and the forks will be far too spongey. You do have a little wiggle room to personalize the fork suspension, but you should always stay within the parameters that are set by the bike manufacturer.

Hydraulic Clutch Fluid

Your hydraulic clutch fluid should be treated the same as the brake fluid. In fact, it is often the same exact fluid. Some clutch systems do require a mineral-based oil compared to the regular hydraulic fluid used. The fluid should be a light yellow color. If it is brown, it is time to change the clutch fluid. Always make sure that you use a new bottle when changing the fluid as an old, opened bottle could have degraded by absorbing too much moisture.