we ran the Webras on K&B 500 and they were the loudest meanest 60's in their time............i'd look for a fuel that contains more castor than synthetic and still add 3-4 ounces of castor to the fuel.
Powermaster 10% would be great for the Webra with some Castor added. you won't need any head shims.
it's a good choice for those planes with a 12x6.

as a rule though, no nitro is needed.

read this from the Webra site, and make your choice of 0-15% nitro as needed.

http://www.webra-austria.at/download/en/motor.pdf