Why Some Passenger Car Motor Oils Are No Longer Suitable for Motorcycles: Gear Pitting Issues 2005-32-0088

The new American Petroleum Institute (API) categories for passenger car motor oils have focused on improving fuel economy and reducing emissions. This has resulted in more fuel efficient oils being developed by lowering the viscometrics and by adding friction modifiers. The emissions reductions have resulted from lowering the percent phosphorus (%P) in the engine oils because phosphorus has been found to poison the catalyst in the catalytic converter.

When friction modifiers were introduced, researchers from four Japanese motorcycle manufacturers published the results of their studies (SAE 961217) which indicated that low friction oil can cause too much slippage in starter motor clutches, one-way limited slip clutches, and wet multi-plate clutches. In that same study they reported that engine manufacturers use 10W-30 grade oil to develop new engine technology, and gear pitting was observed with oils of viscosity grades lower than 10W-30 in all four manufacturers' motorcycle engines.

Since that study, the %P has been lowered in engine oils as the API categories have evolved from SG (no %P limit) to SH (0.12%P) to SJ/SL (0.10%P) to SM (0.08%P). The phosphorus from zinc dithiophosphate is known to form a protective film between metal parts which provides protection against wear and gear pitting if the oil film becomes too thin. During the development of a 10W-30 API SJ motorcycle oil, gear pitting was observed when testing was conducted in a CBR600 engine. Since the %P was limited by the API SJ category and the oil film thickness was limited by the 10W-30 viscosity grade, the solution was to use an extremely shear stable viscosity modifier, not typically used in passenger car applications, to ensure minimum shearing, therefore maintaining the oil film thickness in this motorcycle engine. The use of this extremely shear stable viscosity modifier technology therefore will enable lower %P 10W-30 oils, formulated to meet API SJ or higher categories for passenger car applications, to be utilized in the higher shear environment of motorcycle engines.