Should I worry about a dipstick reading that shows oil ABOVE the full mark?

Dear Car Talk

Dear Car Talk | Jul 01, 1996

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.

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