Dear Tom and Ray:

Each time I had my car's oil changed at the dealership, the dipstick showed that the oil level was a quarter of an inch to almost an inch above the "full" level. According to the owner's manual, over-filling could damage the engine. I called this to the mechanic's attention several times, and each time he insisted that the amount he put in was correct. He also claimed the extra oil would cause no damage. Finally, he checked his oil-dispensing gauge and found that -- guess what? -- it was not accurate, after all. He then proceeded to correct it. I still want to know if that previous overfilling could have caused any damage. -- Will

TOM: It's unlikely, Will. It's true that overfilling the crankcase with oil can damage the engine. But in the vast majority of cars, you won't do any damage in the quantities we're talking about here.

RAY: A quarter of an inch on most dipsticks equals a quarter of a quart. And adding an extra quarter of a quart, or even half a quart, won't hurt anything.

TOM: When you overfill the crankcase by a quart or more, then you risk "foaming" the oil. If the oil level gets high enough, the spinning crankshaft can whip the oil up into a froth, like the stuff that sits on top of your cappuccino. And the problem with that is that the oil pump can't pump froth.

RAY: It's like trying to drink from a straw when there's not much left in the glass -- what you get is mostly air. And air can't lubricate your engine.

TOM: Now, there are some cars that are more sensitive to overfilling than others. Volkswagens with four-cylinder engines come to mind. And if you had one of those, and the mechanic overfilled it by half a quart, I'd tell you to have him drain the oil and refill it more precisely. But unless your owner's manual specifically warns you -- in dire language -- not to overfill the oil, I wouldn't worry about it unless it's approaching a quart over.

RAY: And by the way, your dealership should be giving you free oil changes for life. If you led it to discover that it was giving away an extra quarter of a quart of oil with every oil change, think of all the money you're now saving the dealership on oil!