I wonder why almost everyone thinks Indy in american challenge (or whats its name) is best for EXP grind. Its definetly best for grind overall since it gives nice exp and hell a lot of the money. But as soon as ya put your hands on formula GT i suggest riding monza for example. Around 28 minutes (maybe 28.5 with loadings) and you got 33k exp. Quick counting gives you result that riding monza for 5 hours gives you around same amount of exp as 6 hours of indy.
Worth to mention: count loading times. It takes around 5 minutes 20-30 seconds to do whole indy including loadinds.