Which is even thicker. 75w140 is heavy duty hypiod gear oil for end reductions in rear axles. When I was still working as a truck mechanic we pored the goob in rear axles and hub reductions of heavy duty sand trucks. It doesn't belong in a gearbox, and we were warned not use it. Especially not the API GL5 variant, as it is so hideously doped, it will eat away copper/brass based materials (read synchromesh rings). A single crown-pignon gear assembly is able to easily plough through this stuff, for a gearbox it is as much overkill as using gearbox oil as chain lubricant in the primary. But each his own

Absolutely agree on the ATF. I'm sure your stator will love you for the extra cooling it gets from it

 

Gear oil has a different viscosity scale than motor oil... 75w90 is actually pretty damn close to 20w50 motor oil, and the 50 weight engine oil. So long as you don't have something with some slick **** in it meant to be energy conserving, no problem. Particularly with the stock clutch.

https://www.bobistheoilguy.com/wp-co...1/ISOchart.png Click that and poke around... Stuff isn't what it seems event though the W in the middle makes it feel like it ought to be. Then again, you don't ever squirt motor oil out of the little nipple on a gear oil bottle to see how they compare either.

 

+1
I've been running ATF for 50k+ in my primary with no issues.

 

I started using the 75w140 as a Ford tech, it was easily available.. the coments on GL5 and it makes good sense... I can say, I've never experienced trans has never shifted better or quiter.. I'll probably reconsider that..

As for the ATF, I started with it because the HD clutch is so similar to an auto trans multi disc pack. Again, I think it helped the shifting and it's cheaper than primary fluid..

 

 

I drained the primary on the 89 softail (and my 98 fatboy) and refilled both with ATF.
Thanks guys!

 

Been running HD full syn in my primary and trans. My mechanic says that's why my pro clutch has become grabby and it's hard to find neutral.

 

 

I too am interested in the ATF fluid instead of primary oil. I just replaced my clutch (BDL), Spring plated and chain tensioner. A recommendation on what ATF to use? BDL say's not to use synthetic lubricants on their clutches.

 

The clutch material and characteristics are what dictates the blend. Personally, I've always looked at Harley as a very primitively engineered machine, so I never gave it much thought... with that said, I use merc III, because that's what I had. Since the clutches have already been saturated with that blend and additive package, that's what I stick with.. the better suggestion would be to ask the clutch manufacturer what they recomend.