They certainly did not do anything extra except replace the head gasket. You can count on that. It is very curious that after you replaced the O2 sensor, the runner runs rich/lean. Typically then a vehicle runs rich/lean I suspect the O2 sensor as the problem. You are blowing oil and the O2 sensor will get coated with soot, no doubt. However I do like DRTDEVLÆs suggestion. Blowin one quart/750 miles is trouble though. Good Luck.