If manual transmission require GL4 or 5 gear oil how is it a motorcycle gear box, BMW excluded to some extent, can use motor oil and live?
Ostensibly this is one of the significant differences between motorcycle specific engine oil and automotive spec engine oil.
The other difference is the lubrication additive package, in consideration of wet clutches.
As an interesting note, the 5-speed in my '95 Honda Civic calls for 10w-40 engine oil in the manual. The tranny now has over 250,000 miles on it and has no indications of any issues (I've even on the original clutch).the question is why do most manual transmissions require extreme pressure gear oil while some others like Ducatis get away with conventional motor oil. The answer is probably in the design of the gear box