I've seen enough high mileage Honda, and toyota engines opened up that only used a high quality synthetic like mobil1, or castrol that had 7,500 miles, or higher oil change intervals that look super clean, just a golden discoloration on its engine internals. I've also seen engine internals that were done at these quick change oil shops, that look like they were never done. A know a lady that went to one of these places every 3k miles, or so with her pontiac, and needed a new engine at 75k. She kept all records, and went to a lawyer that won her case for a new engine. I can't claim what I know with many synthetics these days, but I can tell you mobil1 extended works as claimed. It's all I ran in my tundra, and try to in my taco, and our prius. I would also recommend 5k oil change intervals if you have no clue what oil is being used. If you do it yourself, find a well known good grade full synthetic....10k mile changes will be no issue.