If you wouldn't put Gear Oil in your engine, why would you want to put Engine Oil in your gearbox?

FYI: HD no longer recommends the "One Oil in All Holes" BS as the best option. They now just say it's OK to do.

 

Amsoil says that the 20w50 is better in your tranny than the severe gear. unless you have one of the newer bikes with a 6 speed.

 

Besides the search, you might scan thru the sticky topics at the beginning of this forum, specifically:
- Engine Oil vs Gear Oil
-What TRANS oil do you use?

 

If someone chooses to use 20w50 in the transmission....so be it, it is YOUR bike.

Gear lube with a GL-5 rating was originally designed for the Harley transmission. Brand, synthetic, non-synthetic, blend, or even grade is not a major concern here.

If I were to develop a major leak all of a sudden out in the middle of nowhere and no fix was viable. Gear lube was not to be found, but the bike still had to be ridden....then I would use just about any motor oil that was available at the time. I'll use a motor oil in an extreme circumstance in the transmission.

 

From Spectro regarding GL1/GL5 oils.

With motorcycles, gear oil, tranny oil..... same difference. GL-1 usually is the formulation for these applications, meaning it is OK to use in a wet clutch or with bronze bushings or synchro rings. This oil will not have the sulphur smell.

Differential oil, that would be a Hypoid GL-5. Some automotive people use the term 'Gear Oil' exclusively referring to a hypoid differential oil. This oil has the sulphur smell.

The GL-5 is OK to use, but the downside to a GL-5 is that it will eat rubber, so if the seal on the gearbox output shaft ever went south, the oil could damage the drive belt.

This is why HD uses a GL-1 at the factory and why we recommend our Heavy Duty Platinum Gear Oil, 75w140, which is a full synthetic GL-1.

Our Heavy Duty Gear Oil 85w140 is a petroleum-based GL-5 Hypoid oil and is also considered a differential oil, and is designed for the Big Twin chain drive bikes. Our new Heavy Duty Platinum 6-Speed Full Synthetic Gear Oil is also a GL-5 made exclusively to work with the helical cut gears on the new 6-speed gearbox. This oil is also in the differential oil category.

You can use any of these oils in your Fat Boy, but, there is a microscopic chance that if an oil leak developed in that output seal while using the GL-5, you would need to park it before you ruined your drive belt.

http://www.spectro-oils.com/motorcyc...on=v&id=33