Subaru EJ20G rebuild

Hello everyone. I am a lucky owner of closed deck ej20g block built as 2.1 stroker with Cosworth forged internals. Initially, it was running on APS400 turbine. I bought the car 4 years ago with this setup hardly believing in what is inside the engine. Luckily it turned out to be true, unluckily it is burning a lot of oil, ~1L/1000km. I was using 2 oils: Motul 15w50 300V as recommended by previous owner. At one point I switched to Penrite 10w50 HPR10 with no visible difference in the amount of oil burnt.

Initially, it was burning around 200ml/1000km which I assumed to be normal for forged engine but it was constantly increasing. No leaks on the engine, turbo was replaced since there was some longitudinal and transverse play on it with some oil leaks. Next turbo used was IHI VF49, the car was retuned after that (back in 2019) but of course oil consumption did not stop. Intake walls were sticky from oil. I was expecting a lot of oil in the intercooler but surprisingly, there was almost none (surface inside was sticky but no drops of oil). The car was giving P030x errors from time to time without logical pattern. I could see gray/white smoke from the exhaust on cold engine and burnt oil smell. White/gray smoke cloud was especially visible when the engine oil was warmed up to working temp of 90 C, stopped for 10-20 minutes and started again. When starting the engine, there was visible cloud from the exhaust.

When the car was retuned, tuner said he is surprised the engine is undamaged since it was running super lean which would explain P030x errors and underpowered feeling of the car. After the tune, the car was much more vigorous with much less knocking. It was tuned to around 300HP (street tune).

At the point when it was burning more than 1l/1000km I decided to take it out of the car - it has done around 60k km since it was built with Cosworth internals by one of previous owners.

The engine is using standard non-AVCS WRX heads RH/LH S20. Right side of the engine is full of carbon/oil deposit (pictures attached) whereas, left side looks clean with almost no deposits (pictures attached). I took right head (with deposits) to experts' workshop for checks and it turned out to be sealed properly - no coolant or oil leaks through valves or any other problems. It has standard for Subaru heads micro-cracks next to spark plug threads but they do not cause any leaks. I am yet to test the left side head next week (the clean one).

I have removed one piston from the right side (dirty one) as I was expecting that oil consumption is caused by worn/damaged piston rings. Unfortunately, they look good and there are no cracks in ring lands.

I am a little out of ideas at this point. My next theory is that scratches on cylinder walls could cause that oil is not 'scrapped' by oil ring which is burnt in combustion chamber. However, similar scratches are on both sides which is contradictory to how cylinders look like - right is full of deposits whereas, left is clean. Additionally, someone building the engine in the past, made a cut in piston (photo attached) to allow for oil squirters. Is that okay? I think it could cause top to bottom imbalance and 'wobbling' of the piston in the cylinder causing those scratches visible in photos - correct me if I am wrong.

I am looking for help on what could be causing such extreme oil consumption? Why could there be such difference between left/right sides? -

I am also attaching more pictures. You will see pitting on piston crown which exists on all 4 of them. Piston to wall clearance is quite big in TDC but after watching Andre's webinars it looks to be okay for forged piston. Piston deck height is 0.9mm above block surface and head gasket is cosworth 1.1mm.

Attaching more pictures from right side as they were not uploaded.

The way you described the symptoms it's typical valve oil seals failure. The best way to observe it is to let the engine idle for a few minutes and press the gas pedal quickly. The gray cloud will come out for sure if seals aren't working properly. The other way to get the same issue is when the aftermarket camshafts are installed and lobs are too big so they can cause oil seals not to do thier job properly letting the oil inside of combustion chamber. But since you have the issue only on one side of the engine I would not think that's the case here...

Thanks, appreciate quick response. Valve oil seals was my first guess - especially that it happens on one head which has combustion chamber clogged with deposit. Unfortunately, it is perfectly fine. Let's see what the workshop guys say on the left head which has clean cylinders. I would expect the burning oil will leave deposit we see in right head rather than clean cylinders like on the left head right?