i just found out that the po took the breeather on my 84 12 bolt off! i need to do an oil change on it. what should i put in and do i need a new seal or reuse old seal or silicone or silicone and old seal? thanks
and if you fear contamination I would yank the cover. You will need a new gasket or you can try ultra black RTV. The old gasket will get destroyed. Gasket vrs RTV is kinda like ford vrs chevy.....its all preference.
I'd go with 80w 90, and pull the cover, replace the gasket. Use a little black RTV on the gasket to make a good seal.
And since you're doing one axle, you might as well do the other. The minimal time and effort will be worth it.