THREAD DRIFT ALERT.... Let's get back to topic for G100UL not EFI's, by order of self appointed internet hall monitor. ha ha.
A couple of thoughts come to mind-
1- With “No approved alternative” being pretty much the only remaining argument for legal availability of 100LL, how much time will pass before it becomes illegal, especially here in California? Might that motivate a rather speedy transition?
Yep there will be pressure to transition quickly and before availability, like the Car EV mandate. Three airports in California are reported to have already made moves to ban 100LL. We shall see. I hope there is steady progress in producing and distributing G100UL. Since they are interchangeable and can be mixed that may help.
2- I wonder if we will be seeing the return of semi-synthetic(or even fully synthetic?) aviation motor oil availability.
Synthetic aviation oil BLENDS are still in use. The PURE 100% synthetic oil of the past that failed badly in aircraft engines, likely not. Yes tetra-ethyl-lead (TEL) would build up because synthetic oil was a poor solvent, can not hold LEAD in suspension, and after 600-900 hours the rings and valve guides would get "glued". Another issue was full synthetic oils could not handle the high head / exhaust valve temperatures.
I get your point without tetra-ethyl-lead (TEL) the full synthetic may now work. I think there are still heat and particulate suspension issues. The success of the BLENDED "semi-synthetic" oils would not lead oil companies to go full synthetic. There is some complaints about blended semi synthetics already. Exxon is discontinuing their blended semi synthetic aviation oil.
Many well respect aviation mintenance experts swear by traditional single grade base oils for aircraft engines. O
f course ALL OILS have synthetic chemical additives. All oils can be considered semi synthetic, except pure mineral oil used for break in. It is just a matter of percentage of base oil to synthetic additives. All "full" synthetic oils (even for cars) have some base oil as I understand it. The exception is Jet turbine engine oils. They are 100% synthetic. Jet engine oil is cleaner sans combustion products. Although the oil gets hot, it does not cook the oil like a piston engine exhaust valve guide does.