Hogwash, there’s no reason why a 4 cylinder can’t use synthetic oil. There’s no reason why you can’t use it in your Camary if you choose to do so. Now you probably won’t see any huge benefit in using synthetic oil. But you won’t hurt anything. I would look for another mechanic if I were you, next thing you know he’ll be wanting to de-coke your valves and revulcanize your tires.