Checking oil, when hot or cold?
 

I have never given this much thought before and maybe there isn't much difference, but when should one check the oil level?

It dawned on me that for transmissions and power steering fluids, there is a "cold" and "hot" level. Obviously when these fluids get hot, they expand, hence the different level marks. Does this apply to engine oil?

I have always been in the habit of checking the oil when the engine was cold. My theory is that when I do an oil change, I put in "x" amount of cold oil. Also with the engine being cold, I knew that all the oil was in the pan. Yet, every owner's manual for every car I have had says to check the oil everytime you get gas, ie; when the engine is at operating temp.

Again, maybe there isn't much of a level difference. Just thought I would pose the question. I suppose for the purposes of tracking oil usage, that it doesn't atter as long as one checks the level at the same time ie; cold or hot.

Thoughts

The reason for checking oil hot is the expansion you mentioned and that the sump has enough reserve to keep feeding the oiling system without interruption after the oil pump has fully purged the oil galleries.

When I do a cold check I get a slightly high level - abt 3/4 of the dipstick, but when I do a hot check I get 1/2 dipstick which is perfect, I have always maintained all my cars at such engine oil level, unless it was recommended by the manufacturer to a special mark on the stick.

MB suggests checking either HOT or COLD. (in owners manual)
Either way they suggest allowing the engine to sit for at least 5 minutes to allow
the oil to return to the oil pan.

I always check the level on a hot motor & make sure NOT to overfill.

I too check my engine oil when the engine is warmed up, but not overheating. Just at the operating temp.

If I check it when it's cold the level goes up.

It's a good idea to know what the quantity difference is between the max and min marks on the dipstick. In my 300D W123, the difference is 1 quart. On my 190E W201 (2.6), the difference is 2 quarts IIRC.

hot, been sitting for a few minutes and only add oil IF the level is at the low mark

-no sooner or later than that.

You will really screw things up overfilling the engine.

which brings up the question: How much room is there for overfills?

1/2" the length of the pan?

As Randy said, for your E320 it's specifed to read the dipstick about two minutes after shutdown - about the time it takes you for the service guy to check at a full service station.

Also, Mercedes reduced the specified oil charge from 7-1/2 liters to 7 liters for your E320 M104. This would be around the half-way mark on the dipstick. It has something to do about the original capacity listed in your owners manual as being too hard on the seals. So don't even think about putting in excess oil in the crankcase.

What does 7 liters equal in quarts? For the last 40 thousand miles worth of oil changes, I have been dumping in 8 quarts. Technically it's supposed to be 7.9, so I have been adding an extra 1/10th of a quart. What's that, 3 extra ounces.

7 liters is roughly 7 1/4 quarts.

This question was definitively answered 10 years ago by the legendary Magliozzi brothers:

Toledo Blade - Google News Archive Search

I prefer to check the oil when it's cold. Of course I mean when the outside temperature is cold.;) When it's 105 out I don't open the engine cover for anything!


It doesn't really matter. 1/2 qt. one way or the other won't make any difference. Keeping an eye on the oil level is the important thing.