Which gear oil in the diff?
 

Can anybody recommend which gear oil/weight to use? It is for my 67 El Camino, GM 12 bolt, posi with 4.10 gears. Sorry for the noob questions, but which oil, weight, oil additive for the posi, how much will it hold and should I use RTV instead of the gasket? I read somewhere that synthetic should not be used.

I've run both regular and synthetic in the Chevelle, Cobra and Explorer in 75w-90 and I don't think it really makes a difference which you run. Just make sure that you do add the friction modifier. I've been able to find it at my local Kragen but if you can't both Chevy and Ford dealers carry it but they charge more. I've also used RTV instead of a gasket and it didn't leak but I let it cure overnight before I added the gear oil.

I like the lucas stuff, no additives needed. I use a thin layer of rtv over the gasket and let it cure.

I believe it holds 2.45 quarts,
If interested I can supply you with some AMSOIL SEVERE GEAR® (75W-90).

PM me if interested.

-Kyle

Been using Valvoline 140w for over 30 years. The rear end has over 350,000 miles on it. Used GM posi additive last time---Ended up using 2 packs of additive.

Thanks for all the input guys. I used Valvoline 75/90 wt and GM additive...Runs great...

Use the Ford friction modifier, it's better than the GM stuff. Redline has a good friction modifier too.