After a couple of sessions at track day I saw oil splatter on my rear rims. What the heck is that I thought?
Brake fluid? Diff oil? Some Krown from the fall maybe?

It didn't stink too bad so thought the brakes started leaking a bit (I wasn't nice to them that day). Figured I'd keep an eye on it and watch the fluid level until I got home.

Pulled the brakes apart yesterday to find nice dry callipers but wet in around the abs rings on both sides. Axles seals I thought, I hate doing axles seals! Then remembered ZR mentioned he had replaced a few diff vents lately on S197s.

Sure enough the vent was full of crud and plugged up. A little brakes cleaner and some air and it was good as new. Won't know for sure until I'm out on the highway but I think the vent wasn't doing its job so once heat got into the diff the pressure blew past the seals and carried some oil on the the rims.

So the morale of the story is if your car sees it's share of dirty roads, clean your diff vent every once in a while.