Oil Choices for a 1987 Suzuki RM250: Premix and Gearbox Explained
Shop parts for a 1987 Suzuki RM250 Dirt Bike.
Two Separate Lubrication Systems on the RM250
The 1987 Suzuki RM250 is a two-stroke motocross machine, and that single fact shapes everything about how it gets lubricated. Unlike a four-stroke engine that circulates one sump of oil through the crankcase, valvetrain, and clutch, a two-stroke splits the job into two completely separate systems that never share fluid. The first system handles the combustion side: oil is blended into the gasoline so that as the fuel charge moves through the crankcase and up into the cylinder, it carries lubrication to the crank bearings, rod bearing, piston skirt, and cylinder wall. The second system is the transmission, which sits in its own sealed case and bathes the gears and clutch in a dedicated gear oil.
On the RM250, the combustion side is a premix arrangement. There is no oil injection pump feeding the intake tract, so the rider mixes two-stroke oil into the fuel can before filling the tank. That means the oil you choose and the ratio you mix it at are decisions you make at the gas can, not something the bike handles for you. Getting both right is the difference between a crisp-running 250 and one that fouls plugs, gums up its power valve, and wears out its top end early.
What Kind of Two-Stroke Oil Belongs in the Fuel
Two-stroke oils are not all created equal, and the labeling on the bottle tells you more than the price tag does. Look for an oil rated JASO FD or ISO-L-EGD, which are the current standards for high-performance air- and liquid-cooled two-stroke engines. Those ratings indicate the oil is formulated to burn cleanly, resist ash and carbon deposits, and still provide adequate film strength under the heat and load a motocross engine sees.
You will also see oils described as mineral, semi-synthetic, and full synthetic. Mineral oils are the most affordable and work fine for casual trail riding, but they tend to leave more carbon behind and can smoke heavily. Semi-synthetics split the difference. Full-synthetic and ester-based racing oils cost more, but they hold up better at the sustained high rpm and cylinder temperatures that a 250 motocross bike generates, and they generally burn cleaner, which matters a great deal on an engine with an exhaust power valve.
One common mistake is grabbing a TC-W3 outboard oil off the shelf because it is cheap and says "two-stroke" on the label. Those oils are blended for water-cooled marine engines running at steady, lower temperatures, and they are not the right choice for a high-revving dirt bike engine. Stick with a motorcycle-specific premix oil carrying a modern JASO or ISO rating.
Premix Ratios: What Is Typical for a 250 Two-Stroke
Premix ratios for two-stroke dirt bikes generally fall somewhere between 32:1 and 50:1, with the exact figure depending on the engine, the oil, and the riding conditions. For a machine of this era, ratios in the 32:1 to 40:1 neighborhood are the most commonly cited starting points, and many riders of 1980s motocross bikes settle on 32:1 as a safe, well-lubricated baseline. A 32:1 mix works out to roughly four ounces of oil per gallon of gasoline, while 40:1 is about 3.2 ounces per gallon.
Ratio and jetting are tied together, and that is where riders get into trouble. Adding more oil makes the fuel mixture richer in oil but also slightly leaner in gasoline, which can affect how the engine runs. Going too lean on oil starves the crank and piston of lubrication and accelerates wear, while going far too rich on oil can cause heavy spooge, fouled plugs, and carbon buildup on the power valve. Pick a ratio, jet the carburetor to suit it, and stay consistent rather than changing the mix every weekend.
Transmission and Gearbox Oil for the RM250
The gearbox on a two-stroke motocross bike is a separate service from the premix, and it is easy to overlook because the engine and transmission share a common case on many machines. The RM250's transmission and wet clutch run in their own oil bath, and that oil needs to be a motorcycle-specific product rather than a generic automotive fluid. A conventional or synthetic motorcycle gear oil in the 10W-30 or 10W-40 range is the typical choice for this class of bike, and it should carry a JASO MA or MA2 rating.
That rating matters because the clutch is a wet multiplate unit that shares the oil with the gears. Automotive oils labeled "energy conserving" contain friction modifiers designed to reduce drag in car engines, and those same additives can cause a dirt bike clutch to slip under load. A JASO MA-rated oil is formulated with the friction characteristics a wet clutch needs, so engagement stays positive and the plates do not glaze.
Gearbox oil in a motocross bike works hard and gets contaminated with clutch material, so it is worth changing on a regular basis. Riders who race or ride in dusty conditions often change it far more frequently than a casual trail rider would.
Why the Correct Oil and Ratio Actually Matter
The consequences of getting this wrong show up in predictable places. Running a low-quality two-stroke oil or a ratio that is too lean for the conditions leads to scuffing on the piston skirt, accelerated ring wear, and eventually a crank or rod bearing failure that turns a fun weekend into a full engine rebuild. On the other end, an oil that does not burn cleanly leaves carbon deposits that build up on the piston crown, the cylinder head, and the exhaust power valve, which can stick the valve and rob the engine of its midrange hit.
Spark plug fouling is the most immediate symptom riders notice. A plug that keeps loading up usually points to a rich mixture, a ratio that is heavier on oil than the engine wants, or a jetting problem, and chasing it with a hotter plug instead of fixing the fuel and oil mix just masks the issue. On the transmission side, the wrong oil shows up as a dragging or slipping clutch, notchy shifting, and metal debris in the oil when it drains.
Confirming the Exact Spec for Your 1987 RM250
The guidance above reflects what is typical and widely accepted for a 1980s 250cc two-stroke motocross bike, and it will get you riding with confidence. For the precise numbers, though, a quick check against the owner's manual or a conversation with a Suzuki dealer is worth the two minutes. That is where you will find the factory-recommended premix ratio, the specified gearbox oil viscosity, and the correct fill quantity for the transmission case.
If the original manual is long gone, a dealer parts counter or a reputable shop that services vintage motocross bikes can confirm the same details. Once you have those numbers, write them on the inside of your fuel can lid or tape them to the wall of your garage. A consistent premix ratio and a fresh charge of the right gearbox oil are two of the cheapest forms of insurance you can buy for an engine that is now closing in on four decades old.
Related Shopping Categories
Shop Oils and Fluids for a 1987 Suzuki RM250 Dirt Bike.
Shop Oil for a 1987 Suzuki RM250 Dirt Bike.
Shop Air Filters and Oil for a 1987 Suzuki RM250 Dirt Bike.
Shop Transmission for a 1987 Suzuki RM250 Dirt Bike.
Shop Oil Filters for a 1987 Suzuki RM250 Dirt Bike.
Disclaimer: This guide is provided for general informational purposes only and is not intended to replace your dirt bike's official owner's manual. Always refer to your manufacturer's documentation for model-specific instructions, torque specifications, safety procedures, and maintenance requirements. If you are unsure or inexperienced, consider seeking assistance from a qualified mechanic or technician.
