Diff oils...
It's a sensitive subject. If I ever want to fire off and endless debate at my local track, I bring up the question of diff oil setups for our track, which is a combination of clay, astroturf and dirt with a concrete main straight!
The common setup advice tends to be 5-7-1 or somewhere very close to that. As Plaidfish said, the variations in setups dicussed here (and at my local track) are so close!
Fine tuning is the hardest part, but I think the 5-7-1 is an excellent reference point for most tracks.