I doubt the cars deviate at all from the numbers given in the Likely Opponents list, although that only shows power and weight, and PP.
Some AI cars are more tuned than others, but if they did that to make for a more balanced field they failed miserably.
I imagine Sunday Cup cars are all stock, but right from the Beginner events Vitz race, some of them are tuned up.
They don't seem to have fresh-oil bonuses, but they can vary widely. For example, in whatever the first GT race car series is, most of the cars are de-tuned massively. I saw the Camaro LM in the list, waited until I found one, and then discovered that I had something like 150 more horsepower above what the one I was racing against had.
