Most internal combustion (that is, non-turboprop or turbojet) aircraft use 100LL (100 octane low-lead) fuel. Here, up to 0.56g/L of lead content is used as an o

Most internal combustion (that is, non-turboprop or turbojet) aircraft use 100LL (100 octane low-lead) fuel. Here, up to 0.56g/L of lead content is used as an octane booster/anti-knocking agent. Many attempts have been made to remove lead from Avgas but it's fairly difficult since the substitute needs to work with the vast majority of current engines and it needs to be easy to produce and distribute in large quantities around the country. Given the much smaller demand for 100LL (primarily used in general aviation) compared to Jet-A used for turbojet aircraft there hasn't been a big push.