I have a borg warner 9 bolt with 45,000 miles. I changed all the seals since one was leaking a bit. now I need to add new oil. and would like to change it to synthetic. what is the best oil and additive combo for my 9 bolt?
the other issue I have is that the filler plug is a bit craked. is there a place where they sell them or should I just add a bit of silicone to the plug?

 

I have the Borg Warner rear in mine and I went with Royal Purple 75W90 and have had no issues. I did not have to add the friction modifier either with that one. For the plug you should be able to find one at any auto parts store I believe.

 

I used the same Royal Purple oil as Red Dragon and as for the plug, the part number is 10033568, so you should be able to just Google it and choose the one that fits your budget best.

 

There was a recent thread here about the filler plug, you might search a bit for it

I used Amsoil 80-90, though at this point it may not make much of a difference what you use now depending on the mileage, the cones might be bottomed out.
Also, the service manual says not to use friction modifier unless the rear chatters, just an FYI

 

TA rear cover fixes the plug issue and gives you a drain plug.

And yeah the cone type posi is straight hot garbage. Do yourself a favor and put a Torsen in it while it's out.

GD

 

Something to consider

modified 9 bolt diff. cover