I'm not worried about it, it is for my friend. I just thought the mechanic that told him he couldn't mix the two was dumb. I didn't see how it could possibly fuck up your motor. And if synthetic "cleans" out sludge and would then block up the engine, would the GTX "anit-sludge" do the same thing?
But the main question was can you mix them and/or switch without any mess ups.