In the US we use the average of motor (MON) and research (RON) octane numbers. In general the RON runs 8 or so points higher then the MON so what is 91 on the pump is usually 94 or 95 RON. Most octanes you read in a manual are RON. That's why they say 95, yet work fine with the 91 you can buy here.
You can do some DIY octane booster with some Xylene from Home Depot. This is likely the main component of octane booster anyhow. It is about 117 octane and it doesn't take much to add a point. It is higher density than gasoline so it slightly richens the mix, but it's not too significant unless you start using double digit percentages.