Hallo!
No problem at all....ran one of my OS .26 FS to day!
I´m going to use my engines (two OS .26) in a Marutaka DC-3.
I use15% Motul "Micro" all syntetic oil and 5% nitro ( mix my own fuel). RPM was 10.000 rpm using a 10x5 Master airscrew antic prop.
Bu it´s a little bit akward to get to the glowplug when the engine is inverted...so why do you want to mount your engine that way........Its allways better to mount the engine uppward or tillted 90 degrees side or ...canted somewhat from a fully inverted position...!

Regards!
Jan K
Sweden