The S-1 Challenge thing is bull shit. Forget putting a tire or two in the grass, if you hit the damned white line on the right in the first right turn, you're off course and get to start over.
I liked that one myself. I even went back and found some more time. Now S-4 and S-5 had me banging my head on the wall for weeks.
Car suggestion for S-5 please. The fast Nissan only goes two laps on fuel and the Audi kills me on tires because the fucking game doesn't seem to let me NOT change tires when I stop for gas.
S-5 is the Veyron TT.
Sounds like you're doing a mini-enduro, maybe Le Sarthe. I think I used the R35 Touring Car(GT5 DLC), and maxed it out. If I remember correct, I got lucky with the leader pitting on the last lap, and handing me the win.
Also, the Red Bull Standard championship is a pain in the ass. My main problem with this was Trial Mountain, or in this case Trial Patience. I finished 2rd in the championship twice, once by only 5 points. So I figured out, I had to win at least 3 of the races, and no worse than 2rd for the other two. A 3th would result in a tie, as the same guy always finishes 2rd to you/me, or wins if you don't. I restarted this aggravating shit so many times, I was taking short cuts like crazy at Silverstone just to get it done, and Trial Patience was kicking my ass. It has basicly taken 2-3 days to win this damn championship. The AI are dicks. On the bright side, all the re-runs of this, I've won millions in credits. Bastards.