Going in to change the differential oil as unknown when last serviced. I know it's the original (67) rear end and when jacked up, the wheels rotate the same direction. That's about all I know. Calling around to parts stores, the spec looks up as 90w gear oil and I'm pretty sure friction modifiers need to be added as it probably uses clutch packs i.e. posi-trac? Finding a gasket seems challenging so I'm thinking Ultra-Black or "The Right Stuff" silicone will work?

Any tips or tricks? The stores only stock multi viscosity 75-90w or synthetic straight 90w. What do you recommend for fluid with old-time diffs in a modern product world?

Thanks for any replies, the parts guys just scratch their heads.


Edit: (oops) after searching this site I found most answers. Looks like three quarts of 80/90 non-synth with 4 oz. of friction juice. Now I'm trying to figure out why the oil level is higher than the fill plug? I removed the only plug I can find on the passenger side of the diff. It's a 3/8", square drive and slightly higher than the halfway point on the diff casing. When I backed it out oil started running down the side. Is there another fill plug? How can you fill past the plug opening? At least the oil is very clean (yea, someone has changed it recently!)

 

That is the correct plug, and only one. That is how full it should be, so that the lube starts to just run out when taken off. Who ever filled it before maybe the car was not level, like the did it on the driveway with a slant, and if you checked it level, some came out, that is normal to have some come out. If you think there is to much, put the car level and let it drain out till even with the fill hole.

 

The local NAPA will have the correct gasket or can get it. You don't use normal silicone on the gasket, they make a special silicone that's compatible with gear lube, I believe it is green. I always use Ford friction modifier and the ford dealer always has it in stock. You also don't need 3 quarts, 2 is what you need.