All oils are a synthetic blend, no kidding. From what I know (research, etc) synthetic oil leaves a yellow film on your engine parts. A film that conventional oil cannot penetrate. So in essence the conventional oil, when switched back to, only lubricates the film and not the actual piston, crank, etc.. As far as Valvoline's claim, in the words of Chris Farley in "Tommy Boy", "If it's the guarantee that you want, I'll take a dump in a box and slap a guarantee on it". I don't know about Valvoline, I will tell you (again) that ALL motor oil is a blend. I read about Mobil1 using a BMW 325i for a million mile test. They ran the car (indoors) for 1,000,000 miles. Changed the oil regularly, etc. The room had climate control and they were able to change temp, humidity, etc + they would make the room dusty to simulate deserts, city whatever. Point is they didn't miss a beat. Over 3 years later the car performed awesome. The spec'd the cams and all moving parts and found them barely beyond factory specs. That is a huge testimony to BMW and their metallurgy as well as Mobil1 oil.