Not all manufacturers follow the same spec as far as hot and cold, up or down. I was thinking an 8 simply because thats what I have in mine. I also run pre-mix at around 50:1 full synthetic. If the plug is too cold, there will be deposits on the nose of the plug. If the plug is too hot, the porcelain will be porous looking, almost like sugar.
NGK plugs get HOTTER as the number gets lower. (See chart below.)
Bosch plugs get COLDER as the number gets lower.
NGK Spark Plug Chart
Bosch Spark Plug Chart
Bosch Plug Spec Chart