If you want to start an endless war on any online motorcycle forum, just ask, “What’s the best oil?” and stand back. There are countless self-described experts who are happy to tell you all about the oil that lets them go 10,000 miles between changes, or the one that increases gas mileage and horsepower by 50 percent. In fact, the differences between motorcycle oils aren’t that great, but if you really want to see a noticeable change in your engine’s performance and lifespan, just choose the wrong oil.
That’s not hard to do in a market flooded with a variety of brands, types and formulations, but when it comes to buying oil for your bike the first rule is simple: buy motorcycle oil, not car oil. All motor oil starts with what’s called the base stock, either mineral-based (pumped up out of the ground) or synthetic (manufactured in a lab). Additives are then blended in according to the oil’s intended use. Engine oils, for example, get detergents to keep the inside of the engine clean, and viscosity modifiers to keep the oil thick when its temperature rises to a point where it would otherwise thin out. Transmission oils get shear stabilizers to prevent the meshing gears from breaking down the oil. The engine and transmission of most motorcycles use the same oil supply, so motorcycle engine oil lacks car oil’s friction modifiers, which can make the clutch slip.
Synthetic oil can go longer between changes, has higher film strength to protect the engine during hard use and circulates faster through the engine when cold. It also costs more than mineral oils—often a lot more. You can save a little by buying semi-synthetic, which is made with a base stock consisting of mineral oil blended with no more than 30 percent synthetic oil. If you’re a casual rider, don’t abuse your bike and look after your oil-change intervals, mineral-based oil will do the job just fine and save you money in the long run. But if you travel a lot in hot weather with a passenger and a trailer, and treat oil-change intervals as mere suggestions, hedge your bets and fill your crankcase with synthetic.
But there’s less wiggle room when it comes to the API (American Petroleum Institute) ratings. The API rating denotes the additive package used in the oil. The most recent is SN, which is backward compatible with all earlier API designations such as SJ, SL and SM. If you have an older bike and you’ve been buying the oil specified in the manual, there’ll come a time when that rating is no longer available. Switch to one of the more recent ratings and you’ll be covered.
Weight, or viscosity, is another worry for some riders. Do they want the weight specified by the motorcycle manufacturer, or should they get something heavier for better protection? There’s little reason to stray from the recommended viscosity for normal riding. Engines are designed with a certain weight of oil in mind and matched to that oil’s flow rate over a range of temperatures. Substantially thicker oil might not flow as easily, depriving the engine of lubrication when it’s needed the most.
The weight on the can or bottle tells you all you need to know: 15W-50, for example, means the oil flows like a 15-weight at temperatures below -15 degrees F (which is why the W stands for winter) and like a 50-weight at 212 degrees. Unless you make a habit of riding in subzero weather, the 50 is what you care about. Most bikes list more than one approved weight, such as both 10W-40 and 15W-50, so you have some leeway if the weight you’re looking for is out of stock.