The almost universal definition of a “drop in replacement” used by PAFI from 2012 forward - - would be a fuel that all or nearly all of the airplanes could use without (other than placards) physical modification(s) to the engine(s) or airframe. (I.e, such as adjusting the timing or altering the redlines, etc.)
By regulatory definition - - a change in authorized fuel found in the “operating limitations” of the aircraft and engine - - is a “major change”. Under the regulations in Title 14 of the Code of Federal Regulations - - any “major change” requires an amended type certificate or a supplemental type certificate.
I personally went to the PAFI management in 2018 and asked them to start the process under the Administrative Procedures Act to amend the regulations. PAFI refused to do that. Instead, they have been continually claiming they were going to find some “alternative” way. There is none under the regulations - - according to every single attorney I have consulted on this subject.
Further - - there is no existing regulatory mechanism in 14 CFR that would allow the FAA to “waive a magic wand” and authorize a “major change” to the airplanes and engines without someone going through the STC process.