The STC part can be confusing. Go to Swift’s website and look up the particular aircraft. Many aircraft do not need an STC. For example, I looked up a 172M model and it says this:
" Ready To Fly!
Congratulations! This aircraft is FAA-APPROVED TO FLY on UL94 Unleaded Avgas!
This aircraft can use UL94 Unleaded Avgas based upon any of the following criteria:
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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.
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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.
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UL94 Unleaded Avgas also complies with the requirements for Grade UL91 unleaded avgas as specified in ASTM D7547."
https://www.swiftfuelsavgas.com/stc/Cessna/172M/any/any/any
The FAQ covers common questions too.