Dear Tom and Ray:

Please settle a little argument my friend and I are having about engine oil levels. I change the oil in my cars every 3,000 miles. I usually let the fast-oil-change places do the work. More times than not, when I check the dipstick after an oil change, the oil level is above the full mark by a quarter or a third of a quart. My friend says that's no problem because mechanics have told him that the engine will just "throw off" any oil it doesn't need. I say it's a problem because my owner's manual says very clearly not to overfill with engine oil. Am I right to worry about these excesses? -- Skip

Ray: Yes, you're right to worry about them, Skip. Of course, it would be much more productive to actually do something about them. But worrying's a good start.

Tom: Engines don't just "throw off" oil they don't need. The engine oil/lubrication system is a closed system (when it's working correctly). It requires a fairly precise amount of oil to do its job, and that amount of "oil capacity" is determined by the manufacturer.

Ray: If there's too little oil, some parts of the engine will not get adequate lubrication and will wear out prematurely. Plus the oil gets hot and breaks down, and burns off even more quickly.

Tom: But too much oil can be bad, too. If the oil level gets high enough so that it touches the crankshaft, the oil will get whipped into a foam, kind of like the filling in a lemon meringue pie.

Ray: And the big problem with foamy oil is that the oil pump can't pump it. It's like sucking on a straw after you've finished a drink. What you get is mostly air. And air doesn't lubricate engines.

Tom: That's why it's important to check your oil level regularly, and top it up when it's low.

Ray: And why it's also important to tell these quickie-oil guys to put the correct amount of oil in your car from now on (or to take the time to let all of your old oil drain out, which may be the real problem). Since it's all they do all day, you would think they'd have it figured out by now. But since they haven't, you really have to check your oil level before you leave the premises.

Tom: And I'd say if it's more than half a quart over the fill mark, I'd make them fix it.

Tom and Ray lead you step by step through the process of finding and buying a reliable used car in their pamphlet How to Buy a Used Car: Things That Detroit and Tokyo Don't Want You to Know. To order, To order, send (check or money order) to Ruin, P.O. Box 536475, Orlando, FL 32853-6475. You can also order online.

© 1996 by Tom and Ray Magliozzi and Doug Berman Distributed by King Features Syndicate, Inc.

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