Opposed 4, too much oil, what are the consequences? (Reopened)

After an oil change by the dealer, after about 140 miles, I found the oil level to be about 1 quart too high. What are the consequences of this? The service writer said something about the flat 4 burning any excess oil … because the excess goes into the back of the cylinder…

2015 Forester…

First of all, you should ignore virtually everything uttered by a service writer.
Few if any of them have any actual mechanical expertise and what they spout is frequently so…off-base…as to be laughable.

The good news is that just 1 qt too much oil probably didn’t do any damage. Just be sure to have the excess oil drained, and to check the dipstick right away after every oil change in order to ensure that this type of situation doesn’t happen again.

"the flat 4 burning any excess oil because the excess goes into the back of the cylinder?"
I almost spit my gin & tonic onto the screen as I laughed at that one.

If you want to see him really sweat, ask him about damage to the catalytic converter from “burning the excess oil”.

Just ignore him.

Service writers consider part of their job to be covering for mistakes made by the shop. Since they usually know very little about how cars work, they just make something up. The first test for a potential incoming service writer is an integrity test. They don’t get the job unless they fail the test. If they cheat, they get hired with a bonus.

Burning excess oil would contaminate your catalytic converter and your oxygen sensor(s) with soot (which is carbon) making them fail and require replacement. It would also foul your plugs, and even leave carbon spots (hot spots) in your cylinders. It could even load up your exhaust valves.

Beyond that, excess oil can allow the crankshaft to whip the oil into a frappe, aerating the oil going into the bearings (which can cause bearing seizure) and even preventing the oil pump from working properly, leaving it “cavitating”. These events mean total engine failure.

There is good news. Most engines will tolerate and extra quart, as it doesn’t bring the oil level high enough to be in contact with the crankshaft. However, if they overfilled your crankcase, they should absolutely correct it.

Now, as to that idiot service writer…

Thanks, that verifies my opinion. Yes, they did drain the excess. Where I was unclear was the difference a flat opposed 4 would make, compared to the normal vertical placement for the cylinders.


In all automotive engine applications the oil pool (the oil in the pan) is lower than the crankshaft. Regular inline engines, “V” style engines, and all others would be subject to the same risks as a horizontally opposed four.

I want to commend you, by the way, on your checking your oil after the oil change. If more people would do that, I wouldn’t have to repeat the explanation so often. I wish I never had to repeat it at all.

A tip of the hat to you.

In a normal upright or canted 4, 6, or 8 cylinder oil would not reach piston and ring height unless the crankcase was severely overfilled.
With a Boxer style engine even a mild overfill could keep part of the pistons and rings submerged in engine oil when not running and possibly have more of a tendency to force oil into the combustion chambers when in operation.

A quart over full is not something to worry about though although it should be removed. The service writer is vaguely correct about some oil consumption but I agree with the others; most of the time service writer comments should be taken with a grain of salt. Sometimes it may require the entire box of ice cream rock salt… :smiley:

ok4450, yes, I think that is what he was talking about. I agree, it should be (and was) removed. Possibly on a steep hill (with 1 qt over) the oil could go where is should not.

Reason was simple, in retrospect. The engine takes 5.1 qt, so they put in 6.

TSM, thanks.

ONE QUART Over wont cause you too many headaches… In fact you could have gone on and never known if it wasn’t leaking out the breather or giving you some other signs of too much oil…when you get to the point of 2 and 3 qts over you will definitely have issues. The oil level will become so high in the pan that it will start to submerge the crank. When the crank is starting to splash thru that oil all sorts of issues will arise…most notably oil leaks. The damage that multiple qts of oil OVER the mark usually cause is that it blows out oil seals and gaskets…making the replacement of said gaskets necessary. However…just one quart too many in this engine and you really shouldn’t have any issues. If it was returned to the proper oil level now and you DO NOT have a steady stream of oil leaking from the engine…Then ALL IS OK. Just drive on as normal and check your dipstick more often to try and spot any oil loss issues. If you don’t see any oil loss…just move on as tho this never happened.


I need to reopen this. After the dealer redid the oil change, I drove it home, and just now checked the oil level (too trusting, I know).

I can’t get a consistent reading on the dip-sitck. Sometimes it reads a quart high, sometimes not. One time one side of the dip-sitck read OK, the other side a quart high. There seems to be a problem with the oil wetting the material of the dip-sitck. This is my first use of synthetic, could that be the reason? Or could the dip-sitck be picking up oil from the tube it sits in?

I can’t believe the dealer put in an extra quart a second time. Anyone know how I can get a good reading? I wiped it off each time, tried 4 times.

PS, car was sitting overnight on a flat surface. 2015 forester.


Ometimes it does take a few attempts to get an accurate reading. Is it a difference hot vs cold etc.

My Independent Subaru Mechanic told me to do two things: check the dip stick in the morning when the engine is cold for a very accurate reading; at the same time check my coolant level in the overflow container, and be sure it is at the “full” line. I do this once a week, and after a highway trip.

Mine is a 1999 Forester with 285,000 miles. Regular maintenance makes it purr.

Try cleaning the stick, sanding it perpendicular to the stick’s length, cleaning it again and see if that makes a difference. My thinking is that the scratches shold have the same effect as honing does in the cylinder walls, retaining oil on the sides of the dipstick.

When it sits overnight, pull the dipstick out once and read it, that is when it will be the most accurate.

When I get ambiguous readings on the dipstick, like it says one thing on one side, and another on the other side, I’ll clean it off, then put it in and leave it alone for a couple minutes then pull it. When I do that – and provided it’s been at least an hour since the engine was last running – both sides will agree. It helps a lot to have good lighting. Best if you can look at the dipstick using sunlight rather than artificial lights.

keith: That is exactly how I do it every Saturday even though it has never used any oil between changes. I think I mentioned in another discussion that a computer error indicated my car had an extra quart installed. The service manager and I checked and it was fine. That taught me to always check before leaving.

I agree with keith also.

I agree with keith. Also, the cold oil is likely to cling better to the dipstick, which will help you see it.

I tried 4 times, wiping it down each time. It was in the morning after the car had been sitting all night. I could not get a consistent reading, varying between full and 1 qt over. As I said, once there was a qt difference between each side of the dip-stick.

I’ll go back to the dealer Monday and have them show me.

George, I’ll try leaving the dip-stick inserted for a minute or two first.


Make sure you try it using direct sunlight to illuminate the end of the dipstick too. Especially true for newly changed oil. Oil that has been in use a while has a lot of carbon in it, and is easier to see.

You could also try leaving the dipstick out all night and getting a reading in the morning.