If you plan to put the game down for a while, think about topping off your vehicles' reserves if they're around the halfway point or so. Oil changing isn't particularly expensive (at least for the lower-end cars), and it would suck to not be able to race because you weren't watching the levels. If you have more cars, you'll still be able to use them while the others get their oil changed. However, cars also have their own individual oil levels to monitor, which will prevent you from using them if things get too low.
Some races require certain classes of vehicle (Buggy, Rally Car, etc). The vehicles you use, and how you manage them, can have a noticeable impact on how the actual races go.