8 3/4 Rear pinion seal leak - repair questions

[72 Challenger]

Hi Everyone,

Looking to put my 1970 Plymouth Duster 340 up for sale in the spring and move onto another mopar. Underneath it is a factory 8 3/4 with a 489 case. There is a slow rear pinion leak I Would like to fix prior to the sale as it's messing up the paint underneath the car.

Is there a preference for pinion seal? I am on Rock auto and see there are several brands, are there any we know that are prone to being bad?

What oil weight are we using in rear ends? Brands, recommendations?

I know about the friction modifier, I will pick one up from the dealer. Some people say the ford one is better, is this true or bogus?

Is there a gasket between the differential and the case? or is that just RTV?

Thanks team!

[jimynick]

I think it was a National seal I used in my 489, but there's lots of choice out there. Theoretically you're to change the pinion crush sleeve when you do this, but I marked the nut/pinion, counted the number of turns the nut made and reassembled it back to my marks and had no grief. Grease the seal's inner lips. Fluid I think I used 85/140. The Ford additive also went in mine and yes, there is a gasket between the "pumpkin" and the case. If you remove the pumpkin, make sure you do a really good job cleaning the mating surfaces. I put a light schmear of "The Right Stuff" on both sides and have zero leaks. Have fun! 

[RUNCHARGER]

I would change it in the car however if you pull it out I also clean the wheelbearings and repack them too. It snowballs....

[72 Challenger]

Thanks guys! I Was going to replace it in the car but I am going to replace the carrier to differential gasket at the same time. I will also count the turns and forego the crush sleeve.

Re-packing bearings is not a huge deal either.