Blew a seal in the rear end

So i blew my front seal where my driveshaft hooks up to the rearend, it has a small leak of rear end dope. I plan on getting to it this weekend, was wondering how long approx. it might take and what dope i should use...i know it takes 80w - 90. But should i go with the 80 - 90 lucas that replaces the 80w - 90? Also, what is the ratio of limited slip additive to dope i need to use?

Car is a 97gt automatic.

Thanks for your time.