I started focusing on getting to lvl 50 when they changed sport mode races to weekly. Instead of sport mode racing I have been earning XP and credits to get the 6 or 7 $10+ million dollar cars. I am now lvl 50, have all the high dollar cars except the $10 mil Alfa and $15 mil Daytona. I did campaign races like Gr1, Gr3, Gr2 and F1 races but I focused on making sure to get clean race bonuses. It's a pretty good challenge to get the clean race bonus at all tracks with the W08, although it will be easier now that PD dubbed the car down to novice level