Assuming that an engine always has enough oil to be lubricated properly but burns quite a bit of oil, can it cause any damage in the long-term because it doesn't belong in the combustion chamber?
I have a 2005 Subaru 2.5 RS. The engine is known to have quite a few issues including failing head gaskets and piston ring lands. The head gaskets on mine were replaced not so long ago. However, I'm burning about 1L of oil every 1,000 kilometers. I'm the type of guy who checks his oil level very often and I don't mind to add oil. Getting those ring lands replaced is not a maintenance I can afford right now.
I've noticed that the oil filter has a small leak on it, probably because it was over-tightened. However, I don't think that I would lose that much oil because of that.
I'm strictly concerned about the motor. I know excessive oil burning can end up in a clogged catalytic converter.