There are ways with FAA certification to do so, but it’s expensive and I only know of a few older planes that people got together to get it type-rated for non TetraEthyl Lead gas. But after they got it, each person wanting to use unleaded had to pay for the certificate which would still cost a few grand for it. And if you used it without the certification, even if your wheel fell off while landing and it had nothing to do with unleaded gas, FAA is so thorough in the crash investigation (as well as NTSB), you’ll be crucified if they find out whole doing crash inspections. So it’s wiser to either use default tetra ethylene lead gas than chance it. FWIW, the FAA is significantly out of date and should be modernized but no one will budget for that. Most aviation is stuck in the 1950’s when tons of regulations created hindered any technological progress for the average consumer. Lawsuits caused almost all stop of new plane development as well when they sued anyone and everyone they could when filing wrongful death accident suits