After reading what I could find about API SJ, SH, SG oil I think I found not the
answer but AN ANSWER????
The Oil Guide
oil grades are not just for the listing of better, modern oil but also more environmentally compliant oil and one thing that may affect BMW's decisions are the two words used in the oil guide "Automotive engines" The LT engine although similar is not an "automotive engine".
Modern Engine oils (API SJ, SL, SM) have fallen pray to environmental compliance!!! Many modern oils have reduced levels of an important friction reducing additive called zinc dithiophosphate or ZDDP. The reduction of this chemical in oil was based on the fact that zinc, manganese and/or phosphates reduce the effectiveness and eventually damage catalytic converters and introduce minute amounts of pollutants into our atmosphere.
The reduction of zinc dithiophosphate (ZDDP) in the oils with API SJ, SL, SM affects friction and wear in high pressure (EP) friction conditions. ZDDP and other extreme pressure additives are surface modifiers. They modify bearing and journal surface characteristics to allow lubricants to develop more "oiliness" (an old term) or lubricity and reduce the tendency to scuff and gall under heavy load lubrication situations. Camshafts in high speed engines definitely fit this category due to the high velocities and accelerations required to open and close valves many times per second.
This is not as big an issue for modern cars as they are engineered to operate with the modern oils like 5w20 that lubrocate with less ZDDP. Environmental compliance is being directed by the EPA on the auto industry and thus the engine oil industry. Automotive oils drive the entire engine oil industery so many of our favorite old school brands of oil may now no longer be as motorcycle compatible as we think?
Motorcycles tend to reach higher RPM's under excelaration then their car counterpart, even the modern high reving smaller auto engines. The LT's engine was designed prior to 1996 and the engine was designed around the use of oils with ZDDP and other friction modifiers. The LT's engine is built to very tight tolerances and friction modifiers may very well be necessary, who knows?
I think that the engineers at BMW are somewhat old school even when it comes to modern synthetic oil. They are not willing to compromise on oil thus their requirement for SJ synthetic oils to also meet the obsolete SH or SG requirements.
Will Mobil-1 15w50 oil advirsely effect my LT, most likely not but BMW is not willing to accept that risk so they stick to their requirement for SJ oil to also meet the obsolete SH or SG requirement thus ensuring more friction reducing additives.
>>>If you think this may or may not be a problem for motorcycles read this link about the impact modern SJ, SL, SM oil is is having on classic cars! Bad, effects!!!<<<