J Schmidt

So I've had my 2006 S for a year and a half now. It came with records of engine and tranny oil changes but no diff oil changes. When I got it I changed the diff fluid, which was nasty and black, with castrol 75w90 synthetic syncromesh oil. Two track days, 3000 miles, and 1 year later I drained that fluid and sent it to blackstone labs. They were great and analyzed the samples the day they got them. They said engine and tranny showed no wear but diff had lots of metal and silicone. I am hoping these contaminates came from the dirty diff case and residual dirty oil, you know, like you have to rise out a beer bottle several times to get ALL the beer out. I filled my diff with amsoil 75w90 and will be draining it this fall. Hope to see better results. Does my hypothesis hold up?

iDomN8U

Go with a 75w110 if you infrequently track, even consider a 75w140 if you track more occasionally.

spets

Most likely it's residue from the previous oil, but hard to confirm with just one sample.

Change it again in another 3k and see what the numbers say.

I also recommend a switch to 110w if you are going to track the car.

zeroptzero

See my reply in your other post.

But if your fluid came out nasty black after a short run of 3000 miles chances are it was cleaning sludge that adhered to the diff case that accumulated in the past from lack of maintenance. The fluid should be pretty clear after 3000 miles, if it came out clear and with lots of wear then I'd be more concerned.