I recently changed back to using mineral 80/90 GL4 oil from a synthetic 75/90. Seems to shift better with the mineral. I read something about the synthetic being too slippery for the synchro rings? Can someone explain that or point me to somewhere I can read? That isn't intutitive to me

This pic shows two rings. One is new with a rough bumpy surface and an old one that is smooth. As I understand it the new ring uses that rough surface to slow down the teeth so the gears can mesh. If it is too slippery the synchro rings can’t slow things down like they are designed to.

Click to view attachment

Thank you

I thought was supposed to be 80/90 gl5?

Yes - GL-5 to protect the ring and pinion which needs an EP (extreme pressure) lube due to hypoid gearing.

@BillJ

901 transaxle doesn’t use brass based synchros which would be the only rationale for GL-4. Even then, the need to protect the ring and pinion (non consumable) would trump protection of brass synchros (a consumable).

Factory manual
Click to view attachment

Note for OP: don’t freak out that you’ve been running GL-4. Just switch back to proper lube.

Noted thank you. I was using gl 5 before. I will find a gl 5 mineral oil

I have been using this one from NAPA https://www.napaonline.com/en/p/SL_SL2473

seems to work well, but we need to remember these gearboxes are old technology and some level of care must be taken