There is usually an engineered factor of safety for products that hit the market. Since I can reason this out to potentially govern a human life, the standard factor of safety is 4 to 5 when human life is involved. (except planes, 1.5)
This leads me to believe my oil should be changed every 75,000 miles.
so this is the correct answer: every 74,999 miles is when you should change your oil.