Im still working on this. Trust me once I have a definite answer It will be posted here.
I had a theory that it was after 600miles because my Bob couldnt keep the car on the track. However after a few terrible races he seemed to recover his Mojo and went back to his usual lap times. At the moment the test car has nearly 2000 miles on the clock. It hasnt had an oil change yet, the oil light isnt on yet, the oil has "looked dirty" since 500 miles. It hasnt had a Rigidity upgrade yet and BOB still looks pretty consistent.
About all I managed to work out in the Maintenance section is that. Oil Gets dirty at 500miles. When power is down an oil change will give a maximum of 5% increase. An Engine rebuild will give a maximum of 10% increase.