Originally Posted by _cy_
... does anyone know if slightly higher compression engine 9.5 to 1 still will run on regular?
You'll likely get good responses on this.
My take is no, not very well... better if you retard the timing.
You might give it a try and see if it pings (which it probably will, especially under load/hill climbs), but much pinging would be quite detrimental so go easy.
Pretty sure our current premium is lower than the factory recommended octane for your R90S but I don't have the operators manual on hand.
in other words, best to run the highest octane available.