Using one of the super-fast Prototypes (Speed 8, Peskies, R8, Minolta Toyota, BMW LMR, GT-One, 787B) with Stage 4 power is "unfair".
In the Sarth 24-Hour, using the '04 Pescarolo, my B-Spec goon put three laps on the Sauber in 10 hours with the speed at "3" (Steady), then pulled further away at "2" (Cruise). When I drove, I was something like twelve seconds a lap faster than the Sauber. That's "unfair", I guess.
The trick is to find a car that is only a match, and no more, for the AI. Our good friends at Polyphony Digital have no idea how to make that a simple process for us, so we have to experiment with various cars, repeatedly testing, entering to check on who our AI opponents are, exiting, re-entering, etc., etc., ad infinitum...
I've found that the Peugeot 905 and Mercedes CLK-GTR are incapable of winning the Sarth races, while the 21st-century cars are incapable of losing. I'll be checking them all, in hopes of finding one that my B-Spec driver and I will have to work hard with to win. This may take a while...