I have never seen an engine harmed using full Syn either. It's just a choice thing. If it came up on sale I would have some on hand. I'm not one of those who believe that small amount of Castor is ever going to save an engine that is running lean or hot. How you tune an engine is a lot more important. I like how nice and clean running a full Syn leaves my planes. When I was running Wild Cat the first thing I noticed was the smell. It's a lot more pleasant smelling then Power Master. I'm also not brand loyal. If I can get brand X 15% for a few bucks cheaper then brand Y guess what one I buy. I even stocked up on some 20/20 PM that came up on sale. Just a choice thing. I have never seen an engine harmed with store bought fuel. I just watch out for the amount of oil content. A lot of brands don't mark the % on the label so I tend not to buy that brand.