The 1966 Mustang owners manual says that the fuel should be 91 RON octane "Regular" fuel for the 2V carburetor and 99 RON octane "Premium" fuel for the 4V. About what are those ratings using today's R+M calculations? I know that the "Motor" rating is lower than the RON, so I am thinking that any 87 or 89 is probably good for the 2V but how about that 99 RON, what pump octane would that be now? Thanks