I had a chance to look at a turbo charged 99 volvo xc70 with 70k. The car looked nice and all but when I opened the oil filler cap, I noticed lots of carbon and when looking through the oil filler hole, the engine just did not look clean with imminent sludge build up potential. It just seemed like the previous owner did not put synthetic in the turbo charged engine. Even my 160K naturally inspired 2.9L I6 volvo 960 engine looked much cleaner when looking through the oil filler hole. Am I making a sound decision just by looking through the engine by removing the oil filler cap? How about turbo charged Subarus that have long oil filler "neck" that you just cannot see through it?