The theory behind that is that there is all kinds of nasty stuff lurking on the bottom of your tank. What your dad din't know was that most fuel pumps pick up gas off the bottom of the tank whether you're on F or E. Now, if the car has been sitting for a looooooooong time, then the gas will actually chemically decompse. This is when you will have sludge in your tank.
"You know, after being around (1991TT) and (mongoozTT) I am beginning to notice that most 300zx drivers are more arogant about their cars than any DSMer..." - '94 TransAm owner