Not questioning anything. I have never used or had the reason to use gear oil for a motorcycle but I am a believer in Mobil 1 synthetic for auto / truck.
Here is the application information for both Mobil 1 gear oils.
75W-90: Applications
•Mobil 1 Synthetic Gear Lubricant LS 75W-90 is SUITABLE for use in modern high performance automobiles like SUV's, Vans and Light duty trucks requiring API GL-5 level performance
•Mobil 1 Synthetic gear Lubricant LS 75W-90 is intended for initial fill, topping-off or refilling differentials, final drives, transfer cases and other gear applications where lubricants meeting API Service GL-5 and multi-purpose or mild EP gear lubricants are recommended
•Not recommended for automatic, manual or semiautomatic transmissions for which engine oil or automatic transmission fluids are recommended
•Where extended service intervals and warranties are required
75W-140: Applications
•Mobil 1 Synthetic Gear Lubricant LS 75W-140 is recommended for use in modern high performance vehicles like sport utility vehicles, vans and light duty trucks requiring API GL-5 performance that require excellent durability
•Mobil 1 Synthetic Gear Lubricant LS 75W-140 is intended for initial fill, topping-off or refilling differentials, final drives, transfer cases and other gear applications where lubricants meeting API Service GL-5 and multi-purpose or mild EP gear lubricants are recommended
•Mobil 1 Synthetic Gear Lubricant LS 75W-140 can also be used in rear axles where API Service GL-4 lubricant is recommended
•Not recommended for automatic, manual or semiautomatic transmissions for which engine oil or automatic transmission fluids are recommended
•Recommended for many limited slip applications. For certain high level LS applications, additional limited slip additive may be required if axle chatter is observed.