I would assume we probably would have heard about valve recession already if there was a huge problem with running unleaded fuels in the typical Lycoming or Continental engines. The autofuel STC has only been around for roughly 40 years and how many thousands of hours have engines been run on it? I wouldn't be surprised if there are other unexpected issues that crop up however, just like with any new product. There is only so much testing that a company can do to prove a product, and that testing is normally done in a controlled environment. Once the product leaves that controlled environment things can change.