Having an increased top speed is definitely a must to win these and makes it much easier. On Fuji, Indy, and other tracks that contain 1 or more long straights where the A.I.s will rev-out, having a higher top speed is your advantage, as that means you can accelerate past them at those points. The races are long enough as well that you could practically stay in line throughout the more technical portions and only pass on the long straights.
On Fuji, for instance, take your time and try not to let the A.I.s run away down the lower side of the track. Don't force any passes and drive smooth. When you get to the front straight, get the car lined up and draft past the A.I.s. They'll redline before the start line typically and you'll still have more to go.
Also, for the later tracks in the series (race 5 and 6 especially), you'll need to focus more on defending your position rather than advancing it. By this point, your oil will be going bad and you'll be losing power. One thing you could do is ensure that you'll have enough points leaving these races to win by default. That means that in the last race, you should have a 100 point lead, ideally. If you don't, you could always wreck the opponent who is nearest to ensure they don't win (and quit instantly so they don't get a chance to regain it). You don't need to win all 6 races to get the championship gold and the prize car, so you could get away with that, get the championship, and go back and win the races later after you've changed your oil.
