The rear differential on my 99 avant wagon is covered in grease, looks like it has a leak but so far no noise coming from it. I went to fill it, but cannot figure out what size bit is used to remove the plug. I am assuming it's some type of hex head, but I'm not even sure of that. Wonder why they couldn't just use a regular bolt head?!

 

Hi, Are you sure that its grease and not gear oil? If its covered in grease, you should check to see if one of your inner CV Axle boots may be torn. If its not grease but gear oil, you may have a bad seal in the diiferential that's leaking.

P.S. I believe that the fill and drain plugs on the diiferential and transfer cases are secured with, "Torx" head bolts. Probably a T40 or T45 size.

 

I am not sure if the grease is gear oil or CV grease. I assumed it to be gear oil because the CV's look fine, and the grease covers the entire bottom of the differential. But it does seem to be thicker than gear oil. Guess I'll find out when I check the oil level in the differential. If it is leaking, is there an additive to stop the leak? I really don't want to have to take anything apart to fix the leak!

 

I believe that the plug is 17mm allen, it is most likely gear oil, with some added road grime to thicken it up that you are seeing. The inner cv on the rear have no boot, and are therefore not prone to leaking. I would suggest you check the level, if it is low, then you need to locate the source (most likely 1 or 2 of the seals) and repair. The seals are cheap and the repair is fairly straightforward.