Something strange happened to my friend's car. Can you find the fault?

[Karabiner98k]

Some days ago, when I saw my friend I noticed some strange noises coming from his car's engine. When I got in the car, I also noticed oil pressure warning light was on. I told him to rev up the engine a bit and the light went off but it illuminated again when the engine returned to idle.
I thought that maybe there is a problem with oil pressure that has happened due to some blockage in oil galleries inside the engine. But I was also not sure about the oil pressure sensing sensor itself. Since it was cheap, he bought a new one and replaced the sensor but that didn't solve the problem. His mechanic told him there must be some blockage inside oil galleries and he should use some engine flush to solve the problem. I also checked inside the valve cover to see if there is any oil sludge inside but found nothing and the valvetrain area was super clean! I checked for leaks around and under the engine and found no signs of any leaks either.

Some days later when he went to a garage to change his car's oil, the root of the problem was revealed! To my surprise, his car had a massive lack of oil (about 3 liters). How can this huge amount of oil disappear inside an engine without any black smoke coming from the tailpipe? I had done a cylinder compression test in his car and all cylinders were ok. So, I'm sure there is no problem with his piston rings or valve seals. There is also no excessive blow-by in his engine.
So, what has happened to those 3 liters of oil?!

The only reason I can think of is that maybe he poured some fake oil inside the engine that had low evaporation point and had evaporated gradually and caused this problem.
What do you think?

 

[Giannis]

What car/ engine are we talking about?

 

[CrunchSlaughtered]

Some days ago, when I saw my friend I noticed some strange noises coming from his car's engine. When I got in the car, I also noticed oil pressure warning light was on. I told him to rev up the engine a bit and the light went off but it illuminated again when the engine returned to idle.
I thought that maybe there is a problem with oil pressure th...

The good compression is not a sign that the oil piston rings are ok. The compression is maintained by the compression rings and semi-good compression rings in combination with worn oil rings give good compression, because the not cleaned from the oil rings oil acts like a seal. But it could be many, many other things. If the car has a turbo and the turbo bearing is worn a lot of engine oil enters the inlet manifold. If the car is a petrol one the easiest way is to check the spark plugs. If they are completely clean the reason should be some leak that doesn't enter the combustion chamber. Also if the car has a TFSI 4 cylinder engine from VAG that could almost be considered a normal oil consumption. Can you specify also what is the interval between the two oil changes.

 

[Karabiner98k]

The car in question is a Peugeot 405 1.8i Petrol. Mileage = 270.000 kms
The engine is NA, there is no turbocharger.

You may be right about compression test result but this car doesn't consume oil between oil changes and has no leaks. If oil rings were worn, it would consume oil. How is this possible that a car with no oil consumption and zero leaks, suddenly starts to burn 3 liters of oil in a few months?!
Oil consumption is something that starts and grows gradually not all of a sudden without any signs!

There is no sign of oil in its spark plugs. They are clean and dry.

I forgot to mention in my first post that when I checked its dipstick recently (when the warning light was on), it was dirty! But it could be due too lack of oil and accumulated sludge in every car's oil pan.

 

[Giannis]

I's suspect a leaky oil fumes return line which has a leak which you haven't found yet. At least that was the problem in my case. This hose run under some plastic covers on the engine, which -once removed- hid a damn pool of motor oil.