I swore I would never do something like this, but as I just hit Lvl's 40/40 on my own (haven't done Nurburgring 24h yet) I thought I would try it to earn some cash to give all my F1 an LM cars an engine rebuild and chassis rebuild. Works great. Couple points for people though:
https://www.gtplanet.net/forum/showthread.php?t=161147&highlight=grinding
1. Check out one of the original threads. He describes in detail how to do it.
2. Make sure you have all the "assists" ON, which minimizes the car bouncing around.
3. If you are using a controller DON'T use rubberbands (or gumbands, as we say here). Take a post-it note (or any sheet of paper) fold it a couple times and wedge it into the space around the edge of the joysticks, then push them "up". ( FYI, you don't need to tape the "X" button--the right joystick is throttle as well). The advantage of this is that you can more easily do fine refinements of steering angle. Set up the race and watch -- if your car bumps square into the rear of a car on the straight, goes to the LEFT of the car, then back to the wall, you are set up properly. (This is at Indy, Like the Wind))
(edit) As you run along the wall on the straights, G-force should be at roughly .4g to the OPPOSITE side)
4. You are going for speed, not handling -- reduce all downforce, straighten out camber, etc.
5. You should be getting lap-speeds at a MINIMUM of 43 seconds, sub-42 seconds if car has recently had chassis/engine refresh.
6. Remember to give your car a break by stopping every once in a while to change oil or do engine rebuild.
7. Turn the volume off. There is no sound more sickening than hearing a $40 million car being scraped against a wall at 450 kmph.

(edit) The best "lineup" I have found for the fasted race / least amount of bumping is the one where the orange 737 is to your front left, and the blue/green/white playstation 2 audi is to your front right. As you pass the first turn a Toyota 7 will be the only car in front of you, which will be passed at the end of the second turn.