The car's oil will not change during the race, even at the pits. Therefore, over the course of an endurence race you slowly lose power. When the race is over your oil will need to be changed, and doing so will usually get you back most of the hp you lost during the race. Hp that doesn't come back after an oil change is lost due to engine wear, and you'll need to overhaul the engine. As mileage stacks on a car, it will lose hp permanently (very, very gradually over several thousand miles).