Stepping toward unleaded, Aviation Week & Space Technology, 15 September 2014, p 10

In US, the FAA has selected four fuels for Phase 1 testing in the lead replacement programme.   Swift Fuels provide two samples, Shell another, and a consortium of BP, Total and Hjelmeo the fourth.   The two most promising candidates will then be selected for aircraft & engine tests in Phase 2.   Complete replacement of lead additives is expected by 2018.   Currently 167 000 general aviation aircraft in US rely on leaded avgas.