Untrue, CivicInLake.
Racing is *very* hard on engines. Every pro-level race car has a full oil change (if not a full engine rebuild) after *every* race. So, for example, NASCAR machines go between 400 and 1000 kilometers between oil changes, F1 cars go about 300km between changes, pro level dragsters go 400 meters(!) between changes, etc. (Granted, in real life, NASCAR engines are rebuilt after every race, and pro dragster engines are considered "worn out" and need to have everything but the block replaced after a single quarter-mile pass(!), but still.)
Racing engines are so highly tuned and put so much stress on everything involved that you *do* get a noticeable loss of power by the end of a duty cycle (i.e., one race). Top Fuel dragsters, for example, make about 8000 horsepower at the start of a pass, but are down into the 5000-6000 horsepower range by the end of the quarter-mile, due to the wear and tear that's put on the engine.
There's also the little fact that the reason you gain horsepower in GT5 by changing the factory oil is, presumably, that you're putting *racing* oil in the engine. Racing oil is very thin, allowing the engine to make more power, at the cost of shorter oil life and potential engine wear issues. After 300km, the oil is going to be full of contaminants and be (relatively) thick and sludgy, costing you net power because of the added resistance needed to move the oil.
24-hour races are a special case, using high-tech "designer" oil that costs literally thousands of dollars per liter, and until recently, it wasn't at all unheard of for teams to schedule oil changes as part of their pit schedule, along with changing brake pads and some other wear-limited items.
Changing the oil every 300km, and rebuilding it every 5000km, is pretty accurate for a GT racer.
