Maybe he has never read an owner's manual? Mine clearly states that synthetics are fine as long as they meet the appropriate grade specifications. I'll bet yours says the exact same thing.

I'm not so sure about mixing a synthetic with a dino oil, though. Or mixing grades of oil. The point of using a synthetic oil is that it doesn't break down as quickly as the mineral stuff. Does it really do any good when only 25% of your oil has that extra margin of protection?