Motor Oil in Manual Transmissions

[Langanobob]

I always run synthetic 75W-90 gear oil in manual transmissions that call for 75W-90 gear oil. I put a lot of miles on my vehicles but still, it seems like I have too many transmission bearing failures.

Somewhere I remember reading about advantages of using XXW-50 motor oil in manual transmissions calling for gear oil. Then recently a mechanic friend whose opinion I respect made the same suggestion.

Are there any good reasons why say 20W-50 motor oil should or shouldn't be used in a manual transmission where gear oil is factory recommended? Warranty is not an issue since the vehicles in question are long out of warranty.

Thanks,

Bob

 

[Quest]

Originally Posted By: Langanobob
I always run synthetic 75W-90 gear oil in manual transmissions that call for 75W-90 gear oil. I put a lot of miles on my vehicles but still, it seems like I have too many transmission bearing failures.

Somewhere I remember reading about advantages of using XXW-50 motor oil in manual transmissions calling for gear oil. Then recently a mechanic friend whose opinion I respect made the same suggestion.

Are there any good reasons why say 20W-50 motor oil should or shouldn't be used in a manual transmission where gear oil is factory recommended? Warranty is not an issue since the vehicles in question are long out of warranty.

Thanks,

Bob


What kind of vehicle/operations are you operating that would lead to "too many transmission bearing failures"?

Unless the factory specifically calls for using motor oil in M/T trannie, normally MTF is designed specifically with helical-cut gears in mind with detergency, anti-foam, AW/EP additives that ordinary motor oil simply doesn't have and that's what you should pursue.

If, at the end of this you still experiencing bearing failures then something is definitely wrong, and switching to motor oil isn't gonna make things any better.


my 2c's worth.

Q.

 

[ARCOgraphite]

You may want to pursue a true 80w-90 GL-4 if there is no hypoid in the transaxle/transmission. My fiat loved 20w-40 HD motor oil as a trans fluid and I had no problems with it, though that transaxle had 'wall clock scale' gears in it. Hypoid EPs can and do chew up bronze and brass bushes and blockers. Also Please do check you pilot bushing for wear to ensure your trans input isnt subject to no-axial loading - though that input shaft bearing is a big one.

 

[cos]

Consider using these excellent MTLs:

Redline's MTL, MT-90 and Amsoil's MTF and MTG.

Peruse the old posts and you'll also find many us mix these fluids (e.g. 50/50 ratio) to get a desired viscosity.

 

[expat]

I don't get real enthusiastic about many products or Additives, but after using a MTL (I used Pennzoil Syncro) I was really astounded by the difference.
I don't know if RL is better or worse, But I would not hesitate to use a MTL.