Q: You recently advised to follow the owner’s manual in choosing a grade of gas. Does that advice still hold if the manual says regular is fine but, for reasons long forgotten, the owner has been pumping nothing but mid-grade over the car’s entire 17-year life? If it matters, the car is an RX330 with about 120,000 miles.
W. K., Lake Geneva, Wisconsin
A: The only harm you have done is to your wallet.