The mogas STC is not necessary. Swift has done better, stating they meet 80, 80/87, and 91 Octane avgas specs, something which I was unaware until I researched the answer to your question. Swift states that their fuel meets the original type certificate requirements of 80 octane airplanes.
See https://www.swiftfuelsavgas.com/stc/ready-to-fly
Swift has a box on an earlier page which asks “Can my airplane run…” Click on that, and enter your mfr/model. The page will return the airframe/engine combination in my case C182M with O470R TCDS 3A13 and returns the page above.
According to Swift I do not need an STC because my engine is an 80 Octane engine. Swift says this on their website:
" * UL94 Unleaded Avgas complies with the requirements for Grade 80 unleaded avgas (as originally specified in ASTM D910). Grade 80 was last approved as an unleaded grade of avgas in 1995, per D910-95A."
and
" * UL94 Unleaded Avgas meets or exceeds the FAA’s type-certificated fuel requirements of both this engine and airframe. The TCDS fuel requirement for both engine and airframe states one of the following: Grade UL91, Grade 80, Grade 80/87, or a minimum octane requirement of 80 or lower."
So, Swift has stated that their fuel meets the Spec for 80 Octane D910.
Because 80 (the red juice) was approved and meets an ASTM spec and was nearly lead free, I used it for many years until it went away.
Because of the mess 100LL made of the bottom plugs and the engine, after about 70 hours or so running 100LL, I bought the Peterson STC for mogas and began running unleaded (or even leaded) mogas. The engine has run nearly exclusively on lead free mogas through two major overhauls without a whimper (4400 hours, first major at 2720 due to cam wear, the second due now and it continues to be a strong engine without compression, borescope or wear metal issues). I’m planning on continuing to run it until a need arises to pull a cylinder or something else suggests we need to open it up.
Swift Fuel does not require a mogas STC for use of its 94UL variant so even the mogas STC is not needed for their fuel in 80 Octane engines.
I suspect that both Lycoming and Continental stopped making 80 Octane engines in their O470 and O300/320/360 engines when 80 went away or there’d be a lot more 80 Octane powered aircraft today capable of using mogas or Swiftfuel. The only wish I have as I fly a fair amount in the middle altitudes, is a turbocharged engine that can use the unleaded fuels.
I note that Swift is offering a one shot “lifetime STC” for their fuels and any future fuels they use. I’m of two minds on this feature because I don’t think I need it for now, but I wonder if I will need it for their future offerings. Right now, I burn both mogas and Swift when there is a choice. 100LL if I have to. I think I talked to a SwiftFuel guy at OSH a few years back who told me that their 94 is simply 100 base stock without added lead, but my memory is hazy on this.