Dear Tom and Ray:
My wife has an '89 Toyota Camry with a four-cylinder engine. Her dad says that you should add engine oil as soon as the oil drops below the "full" mark. I say you can wait until it is down a quart (to the "add" mark). Her dad says that those small four-cylinder engines shouldn't get low on oil. I say that if it couldn't handle being a quart low, then they would have made the dipstick mark at a half-quart or whatever. Who's right about this? Not that it will change her dad's opinion any. — Richard
Ray: I don't think he should change his opinion, Richard. I think he makes a good point.
Tom: I think the one-quart-low mark is a carryover from the days when oil came only in one-quart cans. You opened them with a can opener, and they couldn't be resealed. So oil was — by necessity — added one quart at a time.
Ray: Now that oil comes in resealable, plastic bottles, you can add any amount you want and save the rest for another day. Or for your salad dressing.
Tom: And while you're right, Richard, that it's not a disaster for a four-cylinder engine to be a quart low on oil, it's better for the engine if it's not low on oil at all.
Ray: Think about it this way: The engine can still run and protect itself with three quarts of oil instead of four. But when you're down a quart, you have three quarts trying to do the job of four. So that oil is going to get hotter and break down faster.
Tom: So if you notice that your oil is below the full mark, why not top it off? Other than because it means having to admit that your father-in-law had a point?
(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk website.)