edit:
apparently one doesn't need an STC when Lycoming themselves give a go:
"With the Lycoming engine model approvals, the EASA SIB immediately allows ASTM D 7547
Grade UL 91 use on European Union based aircraft such as the
Cessna 152....Diamond DA40 and DA42-L360; Helicoptères Guimbal’s Cabri G2;..."
EASA approves 31 more Lycoming engines for UL91 fuel | Helihub - the Helicopter Industry Data Source
I know it is a bit late but I hate leaving this uncommented: we should always have in mind that even if the engine is approved for Mogas-use, it does not mean that the airframe is. Fuel lines, tanks, liners and so on might not be approved and deteriorate significantly faster or even dissolute parts which might end up spoiling the fuel. In the best of the cases you might notice higher engine wear. In the worst case you might get an in-flight engine failure.
More about it here:
https://www.diamondaviators.net/foru...180-t3481.html