I've been using the Sauber and R18's for the 24 minute race. Ran about 12 times.
Won every race using RM and 1 stop.
I drive like it is the last 24 minutes of the 24 Hour race, taking care of tires and fuel.
I lift early, short shift and run at 3/4 - 7/8 throttle most of the time.
Keep an eye on your gas gauge, using less than 25 litres at each 3 minute mark.
By the time you pit and fill on lap 8, usually 95 to 98 L, you will have enough gas to finish the race.
Don't battle with the AI, use them to your advantage.
The last race I sat at the line for 20 seconds, to run out the clock, 2nd place was 30 seconds back, but I had enough fuel for another lap if needed.
Here's the R18 tune if anyone wants to try it. Nothing special.
80 - 80
16.45 - 18.28
4 - 4
4 - 4
2 - 3
0.08 - 0.07
-0.05 - +0.03
Brakes 4 - 7
Trans stock at 242 mph
Diff 7 / 20 / 12
100% using High RPM Turbo
DF 450 - 680