Differential/axle questions 69 Grand Prix
I'm working on a 69 Grand Prix with 10 bolt posi with 4.10 Richmond gears. I started on this because right rear axle seal was leaking fluid badly.
I pulled the axle (no C-clips) and after some research, determined it was probably leaking because there is no inner seal - correct me if I'm wrong. This is why I was confused with seal specs (National 8594S) showing it's for a 1.35" shaft, when the outer seal shaft diameter is about 1.68". The good news is I won't have to get the bearing/retainer pressed off/on to replace the outer seal - better news is I won't get THAT done and still have it leak because of a missing inner seal. The axle (old discolored oil film all the way up to the bearing) shows no indication of ever having had an inner seal, though if I clean it up I may see some wear where the seal should contact (axle went back in the car so I can power wash the underside while waiting for parts - gas tank needed pulling/soldering, coil springs need replacing, etc).
First question is if I'm right about it needing an inner seal - I haven't removed the other axle and don't plan to since the backing plate is bone dry (if I did, I expect to see an inner seal). I only have a 67 Pontiac manual, but all looks the same (no C-clip on axles) as this 69.
Second question is about gear lube. I read various opinions here and will stay away from synthetics. I'll need limited slip additive (any preferences?), but am confused on what type/weight fluid to use. The manual (old/1967) says to use GM 1050081 but gives no viscosity spec. There are (75,80,85)-(90,140) weight gear lubes and I'm not sure what to use.
I bought the car like this, and don't know what kind of diff lube was in it (smells like the old stuff). There was no whine/chatter, but interesting is the fluid seemed to separate into clean fluid floating on "dirty" fluid. The car has been sitting for several months, so maybe this is normal. I noticed because residual oil in the bottom of the case was clear on my hand until I got deeper into it. I assume the new gears wore some during breakin and this resulted in fine sediment - the differential probably has about 5000 miles since rebuild.
After all that... please let me know if there should be an inner seal, and what weight diff lube to use. This car is just driven occasionally, though I do get on it hard (383 stroker with beefed up 200-4R - and thanks to several here who helped me learn about/properly adjust the TV cable).