[renhanxue]

In the EU pilot-owners can self-certify their aircraft for unleaded fuel, provided that the engine is unmodified and that its manufacturer has approved the use of Avgas UL 91. Print an AFM supplement page and put on new fuel quality placards and you can release the aircraft to service yourself. EASA has a set of "standard changes and standard repairs" (CS-STAN) that offer simplified procedures for some common modifications and repairs and this is one of them. CS-STANs have been around for about a decade now.

In the US you still need to go and purchase a supplemental type certificate for your specific aircraft type for this, I believe.

GA is a much smaller phenomenon in Europe than it is in the US but that's not because regulators are out to make it more complicated.