Engine Oil: A Slippery Subject

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.

You think car vs. motorcycle oil is a controversial subject? Read our “Tales From the Dark Side: Putting Car Tires on Motorcycles” article, and then scroll down for the comments.

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.

Amsoil Synthetic V-Twin oil
Amsoil Synthetic V-Twin oil is formulated for high-temperature, air-cooled engines and the demands of frequent, short rides.
Motorex Top Speed 4T
Motorex Top Speed 4T provides smooth shifting and gear engagement, excellent clutch performance and low oil consumption for demanding touring riders.

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.

Maxima Syn Blend 4
Maxima Syn Blend 4 utilizes “surface active chemistry” for lower engine temperatures, increased film strength and extended oil change intervals. A balanced additive system minimizes engine deposits while providing complete protection.
Motul Twin Syn
Motul’s Twin Syn uses Technosynthese tech-nology and was developed specifically for V-Twin engines. It is an excellent choice for the long-haul enthusiast.



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.

Spectro 4 Motorcycle Oil
Spectro 4 Motorcycle Oil combines the highest quality base oils with motorcycle-specific antiwear additives, including shear-stable polymers that help reduce thinning.
Lucas Oil's High Performance oil
Lucas Oil’s High Performance oil promises lower oil temperatures, longer component life, less noise and fewer leaks than other oils.



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.


  1. Oil… a very controversial subject. All major brands are supposed to be equivalent but are they? Why does 20-50 Castrol make my ’66 GTO smoke out the tailpipes but other brands of 20-50 don’t? Why is Redline so much more expensive than Amsoil; both are supposedly ultimate hand-crafted lubricants? Are there really different & necessary additives in Harley brand oil than in Golden Spectro, Amsoil V-Twin, or other premium motorcycle-specific oils? If changed at recommended intervals does it really make a difference? Does MT10 and other oil additives really help? hmmmm….

  2. Thanks for the info.My owners manual calls for 20-50? I believe. I have been using 10 or 15-50. Will this have a negative effect on my Triumph? I am in the VA hospital so I don’t have my manual at the moment.

    • Bill, IMO I doubt that using 15-50 instead of 20-50 will have any adverse effect but would not recommend using 10-50 unless in the unlikely circumstance that you are running your bike in a very cold (below freezing) climate; and then you should use a synthetic. Manufacturers specify oil weights for a reason. If the tolerances within your engine’s main bearings are large, a thicker oil has more of a cushioning effect and the thicker oil also provides more resistance to shear; or wiping away, of the oil between moving parts. Once broken in (where a certain degree of wear is desired), I use nothing but premium synthetic oil in all of my engines regardless of age, oil/water/or air cooled, and natural aspirated vs turbocharged. Synthetics flow better than conventional oils when cold (contain no paraffin) and also do not thicken with repeated heat cycles (their molecular chains are all the same length). Conventional oils catastrophically break down at 240 degrees whereas synthetics can go beyond 300. A loaded down Harley pulling a long hill in high gear can reach 230-240 and turbocharged engines often get their turbos red hot. Pre-2009 Porsche 911s, air-cooled airplanes, turbocharged, and any high stressed engine should definitely use synthetics. I like synthetic oil in everything; the added security is worth the added cost. – Just my opinion and I’m sticking to it! –

  3. I have been beating the living daylight out of my 04 ST1300 for the past 7 years with Honda GN4 dinosaur oil in the crank case and have never had any issues. Use whatever is recommended by the manufacturer and sleep easy. These oil threads are boring.

  4. Paul, I find oil a fascinating subject. One of the reasons our cars routinely now exceed 200,000 miles is due to improvements in lubricity and different uses in different climates place different demands on lubricants. Chris may be a skinny little 100 pounder who rides alone while you & your wife always ride 2-up, weigh 500+, & live in Phoenix where it exceeds 115 degrees on a summer day. Conventional GN4 oil may not be the best choice for both of you under these circumstances. Spending a few bucks more on premium synthetic is money well spent and cheap insurance especially if you intend to keep your bike for over 100,000 miles. …and Golden Spectro sure does improve the shift smoothness of my Harley’s transmission.

  5. If articles discussing oil stopped capturing our attention, not only one no one not read them, but even fewer would comment.
    To say the science of oil and lubricity/friction (tribology) is boring, is to say science is boring.
    As for personal preference per one’s bike, each to their own.
    We’ll know the outcome, if the outcome can be shown to be oil related.

  6. i am gonna try a 25w 60 synth/dino blend out here in southern az. wondering if the dino 20/50 is breaking down in the summer heat.. or i have a bad lifter.. but cold my fatboy is quiet.. heat it up.. gets some valvetrain noise that disappears with rpm.. idk oil pump?.. gonna try this thick stuff and see if that noise comes back.. been due an oil change anyway..

  7. onventional GN4 oil may not be the best choice for both of you under these circumstances. Spending a few bucks more on premium synthetic is money well spent and cheap insurance especially if you intend to keep your bike for over 100,000 miles.


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