[sokoloff]

Legally, they need to run a fuel which is approved on the type certificate data sheet for their engine (and the engine-airframe combination).

The recently developed G100UL unleaded fuel requires a change to the type certificate of the airplane (by serial number) and engine(s) (by serial number) to be legal.

Until that supplemental type certificate existed, these airplanes did legally need leaded fuel.

[kube-system]

I am aware, and was presuming that to be covered by the

> Once it's legal

exception above.

[mchid]

No, G100UL is a full drop in replacement that meets the standards off AVGAS which is what is required by the certificate. Fully legal on all engines with no change to the certificate.

[sokoloff]

It does not meet the composition standard of 100LL, which is why you need to buy an STC (Supplemental Type Certificate) from GAMI to use it legally in an aircraft. (It is a drop in replacement from a performance point of view, but does not meet the original type certificate fuel standard.)