I had this as a write up already, this would be a good thread to place it in.
Well, if I just bought a car like that and wanted to assess it and its oil seals, I would do some of the following procedures and think of the following questions:
As far as burning it:
Valve stem seals: Does it smoke on start up? If so it is your valve stem seals.
Rings: If you do a compression test and or a leak down test, you can get an idea of what shape your rings are in. From that, you can sort of gauge the sealing of that area.
Turbo seals: I once hear this mechanic tell me that if your turbo is damp on the outside, then that means that one of oil seals is bad. Possibly if your turbo’s seals are bad, you will get quite a bit of carbon build up on your pistons. I think that is the reason why my pistons had an excessive amount of carbon on them, from oil entering the cylinders.
As far as leaking it:
There are two types of oil leaks: the ones that drip, and the ones that don’t. You can have an oil leak that runs down the engine and sort of evaporates. If you have multiple leaks like this, collectively running down a large surface area, you will loose a bit of oil and won’t even see it on the ground.
Valve covers: Having a leak like that in the valve covers would be that way. The leak would have to run the height of the engine before it reaches the ground.
Cam position sensor: On my car it was very black and gunky in that area. The CPS has an O-ring that shrinks after so many years and heat cycles. A leak in an area like that (high and around other parts) would get everything damp and evaporate rather than leak—if it is a small leak and not gushing out.
The front cam seals: They are also very high and would run in the timing case and get many things wet and evaporate before reaching the ground.
There are also other places/seals that can also contribute to oil loss, but I just named the ones that I suspect the most for "invisible" leaking.
On an old car with drying out rubber gaskets—and so many of them—it can all add up to quite a bit of oil loss without even dripping on the ground.
Those are my thoughts and theory on the matter so far and what I have come up with.
losing oil leak consumption