Thanks for the quick response.
I'm afraid that I was not clear on the car history.
It is a 1989 300SE, runs on LPG most of the time. Gas is more than 5$/gallon in the Netherlands!
I bought it in 1997 with 120K miles. In 1999, with 160K miles it started to use oil and I removed the head. The guides were very worn, and I had them replaced (+ valves and seats) at an engine repair shop. One of the chain guides appeared to be broken and I stripped the engine to replace it. The owner of the shop advised me to replace the rings with "Perfect circle" rings since the pistons were now easily accessible and there was some leakage. The cylinders seemed to be in good shape, the factory hone was still present. The engine runs fine for 30K after this overhaul, and the oil consumption was much less in the first 20K. But now it is going up, and especially with high revs (kickdown) and it starts to smoke (blue) under acceleration.
Since I have it the car has been used regularly and it properly serviced (by me). At the moment I drive only 12K a year, because of the terrible traffic congestion I ride mostly on my motorcycle (lane splitting is allowed here and you can park on the sidewalk).
That I'm worried about the oil consumption is not because of the cost, witch is minor compared to the rest (1600$ road tax etc.) but out of concern with pollution. Half burned oil is very unhealthy.
When it stops raining I will do a compression test and replace the valve seals.