Can you trust a dipstick?

Question:

I have what might sound like a stupid question but I can’t remember if I’ve ever had a car where the proper amount of oil in the crankcase matched the “full” mark on the dipstick. The proper amount always shows above the “full” mark.

Which is correct, the dipstick reading or the amount stated in the service instructions?

Answer:

The dipstick is not a precise measuring tool. It is there to tell you if your engine needs additional oil, rather than to report exactly what is there.

The owner’s manual for your car may specify that your engine requires five quarts of oil. But if the carmaker needed to be more precise, they could say your engine requires 4.795 quarts of oil at 70 degrees Fahrenheit, in which case you would need a thermometer and some sort of measuring container to be able to pour in that exact amount.

However, motor oil is sold in quarts in the United States, and your car’s manufacturer knows that five quarts is sufficient to fill the sump in your engine, so that is what they tell you to put in.

If that is a little bit over the top line on the dipstick it doesn’t matter because what is critical is that your engine is adequately supplied with oil.

As your engine runs, it consumes a tiny bit of oil even when new.

And if your engine goes down a quart, you need to add one. That’s because your engine is either burning oil that is leaking down the valve guides, or going past the piston rings. It could also be leaking out from a faulty seal.

If the oil goes down much more than a quart, there won’t be enough to supply oil to the bearings under certain circumstances, such as when you go around a corner quickly, which causes the oil to slosh to one side of the sump.

And overfilling, if it is extreme, can damage your engine too because it can cause hydraulic lock.

Keep in mind that oil like water, expands when heated.

If you put five quarts in a cold engine, it may read exactly on the mark on your dipstick.

But as you run the engine, the oil heats up and will read a bit higher than the mark.

In short, just make sure there is the specified amount of oil in your engine at all times, and don’t worry about it beyond that.