The Kent engine in my 4/4 has a minimum octane rating of 97 RON. I use a lead replacement supplement and 14 years ago not all services stations had premium fuel so I used the octane boost version Castrol Valvemaster Plus to meet the minimum. Tom Airey, who tuned the engine for me and does many race and rally engines, told me that the racers generally used a double dose of booster giving an increase of about 4 RON, anything above that had no further benefit.
I have no trouble these days getting 97 Octane fuel and I noticed last year that Valvemaster still do a Plus version but it no longer provides an octane increase.
