If the FAA does what is proposed, and you have an engine listed on Lycoming SI 1070AB as able to run on UL91, you will not need an STC. UL91 and UL94 are ASTM D7547 spec fuels. They are not “mogas” or automobile ASTM D4814 fuels which require an STC. Swiftfuel UL94 does not require an STC for grade 80 aircraft and engines. CURRENTLY, Aircraft whose minimum approved fuel grade is listed as Grade 100LL, Grade 100/130, or Grade 91/96 CANNOT USE Swift Fuels’ Unleaded UL94 Avgas without purchasing an appropriate Supplemental Type Certificate. This is crazy as the EU has allowed UL91 for more than 10 years in aircraft with engines on the Lycoming SI 1070 list. In the USA the AIRFRAME needed to be approved as well. Not so in Europe.