What Type of Oil Does a 1987 Yamaha Big Bear 350 4x4 Take?
Shop parts for a 1987 Yamaha Big Bear 350 4x4 ATV.
Start With the Engine Itself
The 1987 Big Bear 350 4x4 is a four-stroke utility quad, which immediately rules out premix and oil injection. There is no separate two-stroke reservoir to keep filled and no fuel-and-oil cocktail to mix at the pump. Everything the engine needs for lubrication goes into one place: the crankcase. That single fact shapes every oil decision that follows.
What makes this machine a little different from a modern quad is that the same oil supply also feeds the transmission and the wet clutch. The Big Bear runs a five-speed gearbox with reverse, and the clutch sits in the same oil bath as the engine internals. So the fluid you pour in is doing three jobs at once — lubricating the top end and bottom end, protecting gear teeth, and letting the clutch engage cleanly. That is why a generic automotive oil is not the right call here, even when the number on the bottle looks correct.
Viscosity: The Weight That Fits This Class of Quad
Four-stroke utility ATVs in the 350cc class are commonly run on 10W-40, and that is the weight most owners of this era of Big Bear reach for first. Yamaha's literature for air-cooled utility machines of this vintage generally lists 10W-40 as the all-purpose choice, with lighter grades such as 10W-30 or 5W-30 acceptable when temperatures stay cold, and heavier grades like 20W-40 or 20W-50 listed for sustained hot-weather operation.
That spread exists for a reason. This engine is air-cooled, so cylinder head temperatures climb higher under load than they would on a liquid-cooled machine, and the heavier end of the range gives the oil more film strength when the engine is hot and working. The first number in a multigrade is the cold-weather number, and it matters more than most riders expect — a 10W flows noticeably better on a freezing morning than a 20W, which means faster oil pressure at startup and less wear on the first few revolutions.
If you ride year-round in a mild climate, 10W-40 covers the vast majority of conditions. If you plow snow in January or run a hay field in August, the temperature range you actually operate in should guide the choice between the light and heavy ends of that list.
JASO MA and the Wet Clutch Question
Because the Big Bear shares its oil between the engine and the wet clutch, the oil needs a friction profile that lets the clutch plates grab. Many automotive oils contain friction modifiers designed to make parts slide more easily, and those same additives can make a wet clutch slip under load. The industry answer is a JASO MA or MA2 rated oil, which is formulated specifically for machines with a shared engine-and-clutch sump.
You will find that rating printed on the back of most powersports-specific four-stroke oils, and it is the single most useful thing to check before buying. Yamaha also offers its own branded four-stroke oil blended for this type of application, and following the manufacturer's own recommendation is a sound default when you are unsure. The key point is simple: pick an oil built for a wet-clutch powersports engine, not one built for a passenger car.
Beyond the Engine: Gear Oil in the Differential and Rear Case
Engine oil is only part of the lubrication picture on a 4x4 utility quad. This machine is shaft driven, and it carries a front differential plus a rear gearcase or final drive, each with its own fill and drain plugs and its own lubricant supply. Those components do not share oil with the engine, and topping them off with engine oil is a mistake.
Hypoid gear oil in the 80W-90 range is the typical fill for this class of drivetrain, though some front differentials use a model-specific fluid tied to how the four-wheel-drive engagement system works. That distinction matters: putting the wrong fluid in a front differential with an engagement mechanism can change how — or whether — the front wheels lock in. Treat the engine oil, the front differential, and the rear gearcase as three separate service items on three separate schedules.
How Work Duty Changes the Oil Story
A Big Bear was built to earn its keep, and hard use is genuinely harder on oil than a casual trail ride. Plowing, towing a loaded trailer, crawling through mud, crossing water, and long stretches of idling all push oil temperature up and pull contaminants in. Water and fine mud can find their way past seals and vents, and a milky, coffee-colored appearance on the dipstick is the classic sign that moisture has gotten into the sump.
Cold-weather operation is the other factor worth planning around. If your quad sits outside and gets started on frosty mornings, the lower first number in a multigrade oil is doing real work for you, and it is a good reason to favor 5W-40 or 10W-40 over a heavier straight-weight or 20W-50 fill during the winter months.
Conventional or Synthetic
Both will work. Synthetic oil generally flows better when cold and holds up longer under sustained heat, which suits a hard-working air-cooled engine and makes it a reasonable upgrade for anyone running long days in the summer or starting in freezing temperatures. Conventional oil costs less, is perfectly serviceable for a machine that sees moderate use, and remains a common choice for older quads. The tradeoff is mostly about how hard you ride and how often you want to change it.
Change Frequency and Confirming the Exact Spec
Severe service — mud, dust, heavy towing, frequent short trips — tends to call for more frequent oil changes than light recreational use, and the same logic applies to the gear oil in the differentials. If you have been through deep water or noticed any milky tint in the oil, that is a signal to change it sooner rather than later.
For the precise viscosity, the exact fill quantity, and the specific drivetrain fluid types for your machine, the owner's manual is the fastest confirmation, and a dealer can fill in the gaps if the manual has gone missing. Treat that as a quick check before you buy, not a reason to put the job off — the guidance above will get you into the right aisle, and the manual will confirm the final number.
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Disclaimer: This guide is provided for general informational purposes only and is not intended to replace your ATV'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.