GAMI chose not to attempt FAA Certification. As I understood it, they saw the FAA certification to be so onerous and lengthy of a process, that it wasn't worth the expense for the questionable results. They chose instead to go via STC and plan to offer a free STC to every engine possible. Does that imply that some engines may not be able to use it? Possibly.
Swift attempted, and is still attempting to go the FAA/ASTM certification. That process has dragged out for nearly a decade. Swift has now chosen to sell STCs for their fuel for $100 per plane and now say they plan fleet wide implementation via STC. Why have both potential drop in replacements had to go the STC route rather than certification? This clearly points to a problem with the FAA and their professional foot dragging.
MOGAS issues. The big oil producers have actively pursued running Mogas off airports. FBOs and fuelers that sell Mogas are threatened with having their supply of 100LL shut off if they don't remove the Mogas sales. I ran Mogas exclusively for 10+ years until I left NM and retired to AR using a 110 gal tank and fueling rig in the back of my pickup.
In AR, Alcohol free premium Mogas is readily available. However, all of the stations in my area now have an injector cleaner additive in the fuel that I have found to cause as much or more damage than alcohol contaminated fuels. (details of the damage and testing on my web site) The carb and engine was fine with it, but it will dissolve pipe dope from the pipe fittings and will break down epoxy resins and any plastics or rubberized fuel lines in the fuel system. The last station in my area that was carrying clean fuel without the additives lost his source of clean Mogas a few months ago, so I have no choice but to switch to 100LL for both of my planes for now. Mogas has always had an uphill battle getting onto airports and local supplies around the country, even at gas stations it is iffy at best.
One other issue with buying Mogas. Most pilots that use it buy at the gas station and haul it to their planes. The fuel taxes paid there go toward the state and federal highway funds rather than the Aviation trust fund. And there are no flowage fees to support the State Aviation fund or the local airport.
100LL environmental issues. I have little doubt the environmental issues to the public from 100LL are so far below the noise level as to be negligible. However, the lead contamination many mechanics suffer continues to go untested, unreported and usually unnoticed. Mechanics routinely exposed to inhalation of lead salts from bead blasting combustion chambers and regular exposure to 100LL fuel on their skin often times have abnormally high lead levels in the bloodstream, well beyond the EPA acceptable limits, and often times with devastating effects. That grumpy mechanic that acts like a mad hatter might just be that way thanks to his chronic exposure to lead. It's only a matter of time until some smart enterprising lawyer decides to bring a class action law suite against the oil companies in behalf of the many thousands of mechanics with chronic lead exposure, that could also spell an end to 100LL and would certainly discourage the oil companies from continuing to produce it.
100LL is a threatened and endangered species. As well it should be. If the AOPA will lead the way to support a transition away from 100LL as rapidly as possible, they will have my support. Had the FAA supported this effort, 100LL should have died out shortly after leaded autogas decades ago.
-Cub Builder