I agree with the others about the oil light. It seems to come on around 189 miles and then you start losing horsepower. I think there's a limit to how much it'll lose, though. I re-ran the Polyphany cup not too long ago. Since that is a 20 lap series, the light came on pretty early, but after a while, the HP decrease stopped at a certain point. (If anyone doesn't know, the car's HP is at the bottom of the screen when you choose "Settings" before a race.)
In one long series while first progressing through the game, I decided to skip the Test Course race (since it's an oil-killer) and save the HP loss until later. It probably didn't make much of a difference.
But, if you think you're cutting it pretty close and the HP loss is going to cost the series, you can always get a tune-up or turbo upgrade one stage above whatever you're using and install it when you start losing HP.
)
