I got an alignment and oil/tranny/diff changes at 11,538 miles (car has about 1000 track miles). Before picking up, the tech wanted to tell me about the alignment results, and then mentioned I was waaaaay past due for a diff fluid change. I told him I was under 12k miles and he was shocked. He said the fluid was about as bad as he's seen from an S2000 (note he's a WRX owner/tuner and the dealership has him do most all work on the S2000's that come in).

Anyway, he guessed I had 40k to 50k miles on the diff fluid, so it might be a good idea to consider changing yours.

 

That is common. I changed it at 19k the first time, and it was dirty with shavings on the plug. I then changed it every 3k 3 times and it cleaned up considerably. Now I'll probably do it every 7k or so.