it usually takes about 8 or 9 laps of NS to feel the difference significantly between 'new' and 'old' oil..... thats why my hot laps of NS are only 8-9 laps at a time
oh, and definately change your oil after any enduro race.
finally... about loosing bhp from the garage ----> race configurations... its my guess that the garage numbers are what it 'ought' to be... but the 'pre-race' numbers are what the game programers think they would be.... sometimes more, sometimes less... the more you play the game, you'll notices this