Hello all, after purchasing my '83 GSL back in February it's finally time to put the car on the road. Soon I'll be in the midst of putting new fluids in all around, but the only thing troubling me is the trans and rear end oil. Redline brand seems to be the unanimous choice here, but that's not the problem. If I'm understanding this correctly, both the trans and rear end call for 80w90 (Redline 75w90 shouldn't be an issue, correct?) which is fine. The issue lies in the specification... the trans calls for GL-4 and the LSD rear end calls for GL-5. Is it crucial that GL-5 be used for the rear end, over GL-4 with the proper LSD additive? I'd rather not have to buy two different oils because the trans calls for 2.1qt... who would want to buy another qt just to have the majority of it sit in the garage and not be put to use.

I hope this is a simple answer. -Ryan

 

Put MT-90 in the trans and 75W90 GL-5 Gear Oil in the diff.

 

The simple answer is that if you're going to do it right and avoid problems with your LSD not working correctly (or grinding itself to pieces), then you get the right fluid for where it needs to go and you keep the extra in the garage to top off - or dispose of it if you don't keep stuff around.

Be sure to get the LSD additive and mix in the correct proportion, too. Good luck,

 

Fair enough. I'm not sure if I miss it on the label or not, but most gear oils don't say if they have LSD additive already. I guess the simple solution to this would be if it chatters, put in the additive. More than likely I'll end up running MT-90 in the trans and Lucas or Royal Purple in the diff.

 

If they don't say they have it, they don't have it.

 

... and most of them WON'T have it - so plan on buying it when you buy the other oils you need. If you wait and listen for chatter you'll never hear it, it'll overheat and it'll be too late and you'll cook your plates.

The little GM bottle of LSD additive was only about $8, IIRC - so makes no sense not to put it in when you already have the fill plug out to replace the rear end oil. Good luck,