DDK - Die Deutschen Klassiker

My US Spec '67 911S engine was rebuilt (according to receipts) in the very late '80s early 90s ( I think it was '89 the documents are at home).
The engine was taken out to 2.2 but retains the early crankcase & I think the heads.
What is the advice regarding unleaded fuel & octane ratings. The previous owner mostly used Shell Opitmax @ 98RON. I have heard good reports about one of the octane boosters available (will need to check which specific one it is).
Wadaya reckon
thanks
Martin

I went through the same thoughts with my 2.8 MFI motor, and have tried 100 RON leaded 5 star (still available through a limited number of outlets), LRP, premium unleaded and standard unleaded and I couldn'y differentiate between them apart from the price! It doesn't suffer from pinging despite running a 10.5:1 CR. Stick with unleaded unless you have a problem