Motorcycle Oil Facts

Motorcycle crankcase oils are specially formulated for motorcycle applications that have wet clutches. Most automotive oils are not suitable for use in motorcycle engines. A premium quality motorcycle specific oil typically has higher levels of anti-wear chemistry such as zinc and phosphorus than some automotive oils do, which are critical in protecting high RPM and high load and engines from wear and especially cam lifters and follower wear.  That is not to say that all motorcycle oils have increased levels of zinc and phosphorus; many do not. 

Over the years, automotive manufacturers have requested oil manufacturers to reduce zinc, phosphorus (an essential element in ZDDP, Zinc Diethyl Dithiophosphate) and other extreme pressure additives from the oil. The primary reason is that automotive manufacturers have requested is that they are extending emissions systems warranties to periods of 10 years/150,000 miles and they claim that high concentrations of these chemical additives were contaminating their catalytic converters and negatively affecting emission levels required to meet their 10 yea/150,000 mile warranty requirements.  There are also a few motorcycles on the market that have catalytic converters, however they are not subject to the same 10-year requirements as the automotive industry for emissions levels. 

We recommend looking at the oil manufacturers specification sheet and performing a comparison. If they won’t provide the data a very simple spectrographic analysis lab test can be performed that will provide the exact PPM (parts per million) concentration of zinc, phosphorus and other chemicals. That is the only way to get accurate information. There is no reason why a reputable oil manufacturers customer technical service department should not have that data (as well as all the other ASTM test data references in this book) readily available for the consumer and if they do not we would be highly suspect as to why not and go out and get the data myself before I started using their oil. 

Another issue with using an automotive type oil in a motorcycle with a wet clutch is that many automotive oils have friction modifiers. Friction modifiers in automotive oils tend to provide increased frictional reduction and offset a portion of the fuel economy lost when the U.S. Government mandated a reformulated gasoline for emissions requirements. However these friction modifiers can be detrimental to proper wet clutch operation and can cause slippage and other potential performance issues such as “engine start failures, a rough running feel and increased fuel consumption due to higher engine stall speeds”. 

It is also important to note that there are automotive type oils that do not contain friction modifiers. You have no way of knowing unless the specific manufacturer tells you. That is why we say it is better to be safe and use a motorcycle specific oil in a motorcycle engine with a wet clutch that meets the proper motorcycle specification as stated in the next few paragraphs.

A premium quality motorcycle engine oil can be stated to meet the requirements of API Service Classification SJ, SH, SG, SF, CH-4, CG-4, CF-2, CF and CD. Note that SH, SF, SG and CD are now obsolete. Motorcycles specifying an oil meeting any of the obsolete applications can use an oil with the newer classification oil because it includes, and supercedes, the requirements of the obsolete service classification.

For example, if you have an old motorcycle that specifies SE service classification you do not have to go out and look for an oil with that service classification (and if you did you would most likely not be able to find any in stores as SE was outdated in 1979). The newer classifications include the outdated ones and are perfectly suitable for use. There are some motorcycle oil manufacturers that will include an older service classification on the bottle to so that consumers can specifically see that the oil not only meets all the older specifications but the newer ones as well. 

It is important to note that as an oil classification is superceded by a newer one, that the newer one provides improved benefits over the older one in such areas as high and low temperature deposits, wear, rust and corrosion for example and is perfectly suitable for use.  All quality motorcycle specific oils should also carry the JASO Motorcycle  Specifications, which are either the JASO-MA specification (no friction modifiers) or the JASO-MB specification (with friction modifiers).  The JASO-MA rating specifies that no friction modifiers are used.  Note that the JASO-MB motorcycle does have friction modifiers, but is designed for motorcycles that specify the JASO-MB specification. If the oil does not carry one of these specifications, even if the other specifications listed above are present, I recommend finding an oil that has the proper JASO specification clearly labeled on the bottle. 

Petroleum or Synthetic?  

On the subject of whether or not to use a petroleum or synthetic motorcycle oil our recommendation is to use a premium quality synthetic. There is absolutely no comparison between the two. Synthetic oil has been proven to resist high temperature breakdown and shearing effects at higher engine operating temperatures for a much longer time period than petroleum oil and as a result allows the oil to stay in the specified viscosity grade much longer. Synthetic oil is more uniform in molecular structure and will reduce internal friction and thus provide for lower engine operating temperatures as well as provide for smoother shifting. 

By measuring the cylinder and cylinder head operating temperatures in two identical motorcycles; one running petroleum oil and the other a premium quality synthetic, an average of 15-20 degree F. temperature difference was measured after a very mild ride and short period of idling. At extreme engine operating temperatures and conditions this value can be up to 35-45 degrees F. lower with synthetic oil. We have received reports of this from motorcyclists that have oil temperature guages installed in their crankcase.

Another report of a customer that  was using petroleum oil in hot summer heat and traffic jams said he would have to shut off his air-cooled V-twin engine because it began overheating. After changing to a premium quality synthetic motorcycle oil his engine temperatures dropped significantly and he no longer had that problem on hot summer days in traffic.

Again there is no comparison between the two and anybody that tells you otherwise has obviously not reviewed documented test data and facts clearly showing that synthetic oil outperforms petroleum oil in every category. 

We have a report of a dynamometer test run comparing a specific manufacturers branded motorcycle oil to a premium quality synthetic oil.  In the first part of the test with the petroleum oil installed in the engine, transmission and primary chaincase the maximum horsepower (Hp) was 68.1 and the maximum torque was 80 lb.-ft.  In the second part of the test the same motorcycle was used and all petroleum oil products drained from the engine, transmission and primary chaincase. With the synthetic the maximum Hp was 69.8 and the maximum torque was 81.3 lb.-ft.  That’s a gain of 1.7 Hp and 1.3 lb.-ft. torque, which is what we consider a significant amount of gain just from changing from petroleum oil to a premium quality synthetic oil. 

Synthetic oil will hold contaminants in suspension longer and reduce any oxidation and provide for a much higher load capacity as well as provide for increased wear protection to critical engine parts such as pistons, cylinders, gears, camshafts and bearings.

Synthetic oil also has superior cold weather performance characteristics and will flow at temperatures that will cause petroleum oil to solidify and can be used for extended drain intervals beyond that of a petroleum oil. Note that during extended drain interval operation you need to change the oil filter at the filter manufacturers recommended change intervals, then install a new filter, top off the oil and your ready to go. There is a premium quality  oil filter on the market that can be used for twice as long as a standard filter, the AMSOIL SMF motorcycle oil filter. 

Synthetics and Slipperiness in Relation to Roller Rockers and Wet Clutches 

There are many reports of motorcycle shops telling customers that synthetic oil is so slippery that is causes roller bearings not to roll and will result in flat spots on the roller bearings. 

This is absolutely not true. Synthetic oil is no more “slippery” than petroleum oil. Synthetic oil is more uniform in molecular structure than a petroleum oil but it is not more slippery and will certainly not cause roller bearings to roll. Roller bearings will roll whether they are using petroleum oil or synthetic oil and the only way the roller bearing on the rocker arm will ever stop rolling is if the needle bearings that support the roller bearing failed.

Every automotive racing engine (as well as many other types of high performance engines) and most of today’s high performance automobile engines use camshafts with sophisticated roller rockers with absolutely no problems of this nature whatsoever.

You also hear motorcycle shops telling customers that synthetics are so “slippery” as compared to petroleum oils (which simply do not reduce friction as well) that wet clutch packs in their motorcycle transmission and even their automatic transmission of their cars and trucks will slip when using “slippery synthetics” even if there are no friction modifiers present. This is absolutely not true. 

Look at it this way. Wet sandpaper removes paint as well as dry sandpaper does. The slipperiness of the water does not impede the sandpaper's ability to function. The same applies to the slipperiness of synthetic lubes in wet clutches.  It is simply not an issue. However, just as rinsing the sandpaper keeps it cleaner longer so it functions better longer, so the synthetic lubricant keeps wet clutch plates cleaner longer so they function better. And, since synthetics are superior cooling agents to conventional petroleum lubes, using synthetics will help wet clutches last longer, too.

Petroleum oils have low resistance to heat and allow varnish and glaze to form on clutch plates, which can lead to slippage and increased heat generation and potential failure of the clutch pack.  Synthetic oil is going to allow your wet clutches to perform better (especially under extreme heat, RPM and load conditions) and last longer than they would with petroleum oil subjected to the same operational conditions.

As a side note in respect to Automatic Transmission Fluid:  synthetic ATF is not more “slippery” than petroleum ATF.  The base fluids, whether or not petroleum oil or synthetic oil, play no direct role in the relative friction levels of wet clutches. The friction-modifying additives developed for petroleum oils work just as expected in synthetic PAO’s fluids.  The longer the fluid resists oxidation, the longer the original frictional properties remain. The superior oxidative stability demonstrated for synthetic ATF’s thereby leads to extended retention of frictional properties. 

Which Viscosity and How Long Can You Use It For?  

When it comes to motorcycles oils the most common choices are 10W-40 and 20W-50.  There is also SAE 50W and SAE 60W oil that is generally specified for older Harley-Davidson motorcycles with Flathead, Panhead or Shovelhead engines. There is also a 0W-40 and a 5W-40 motorcycle oil which is generally used in 4-stroke off-road motorcycles and ATV’s and for machines used in cold weather operation, although a 10W-40 synthetic has an exceptionally low pour point and is also suitable for use in cold weather operation as well as extreme high temperature operation (again, due to the superiority of synthetic oils).

In general, 10W-40 is best for most of the Japanese machines and the 20W-50 is best for V-Twin engines, such as used in Harley-Davidsons.  For specific recommendations consult your owners manual as there are exceptions to this. 

Motorcycle oil drain intervals should be performed at the manufacturers specified oil drain interval, unless you are using one of the premium quality synthetic oils on the market which specifically state that is can be used for twice as long as manufacturers recommendations.

If you have a particular brand or type of oil and you want more exact change interval and performance intervals, other than the motorcycle manufacturers recommendations, then your option is to perform used oil analysis testing over a period of time in order to establish a trend of how long the oil is suitable for continued use based on your motorcycle and operating conditions.  That is the only accurate method that will assist you in determining exactly how well the oil you have chosen holds up over a period of time in service. Oil analysis testing is covered in detail in here.