Knowing the mileage interval alone is NOT ENOUGH INFORMATION.
YOU MUST KNOW HOW IT WAS DRIVEN!
I can see where if a car sees nothing but highway miles in long trips, 7,500 mile oil changes MIGHT work. If the same car, on the other hand, were driven back and forth to the office every day that was less than five miles one way, then 3,000 mile intervals would not be often enough.
There are many variables in this equation, make sure you are aware of them all.