Most manufacturers are leaning towards less viscous lubricants these days for the sake of economy. Witness the fact that manual gearboxes now generally require ATF as a lubricant instead of the previous standard of gear oil. A differential has helical gears in it which means that EP rated gear oil is required due to the angular thrust of the gear teeth towards each other. I would say that a suitable weight of motorcycle gear oil in a diff would be fine but 75 - 140 is too thick in my opinion. All that happens if you use a thicker than necessary lubricant is that there is more heat generated by the shearing action of the gears passing through the fluid. It just overheats the lubricant and the heat breaks it down. Cheers Dan

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