With the GS engine on the work bench, i first cleaned it a bit to prevent stuff getting into the main block. I removed the oil cooler and was not surprised that it was a very clogged and dirty because of dried out non sealing o-rings. The cooler is the second main reason that these engines leak. They are also sealed to the block by two o-rings that dry out.
The main reason for these engine to leak oil are the return tubes that let the oil in the cilinder heads return to the sump. They are fitted with o-rings to seal the tubes to the head and the engine block. The rubber of the seals can’t stand the heat, become hard as a rock and loose their sealing capabilities. On my engine, the o-ring on the block side was still somewhat flexible, but on the (hotter) head side it was totally hard as a rock. It cracked when i tried to remove it from the tube with a screwdriver.