Brian
I developed the noise reduction STC some 30 years ago together with my second generation of unleaded Aviation Gasoline. Lycoming approved my unleaded AVGAS Hjelmco 91/96 UL already year 1995. For using my unleaded AVGAS you do not need any STC in EASA territory. It is a drop in fuel. We have the fuel at about 100 airports in Sweden now for 30+ ears. With the UL AVGAS and the STC for noise reduction it is the most environmentally ?? friendly GA aircraft in the World.