Why 75w90 in tranny?

[cHarley]

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.

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[WaterBuffalo]

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.

[gdkenoyer]

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?

[EVOrider]

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.

[scott63]

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