I use Mystik JT-4 from O'Reilly's in all my customers. Right up there with OEM oils.
https://www.oreillyauto.com/detail/c/jt-4/mystik-jt-4-1-quart-marine-gear-oil/mys1/jt4gearlube
It has a X flow crankcase...looking at carbs, the ones on left feed pistons on the right (2/4/6) and vice versa. Check the throttle plates as cylinder 5/6 can be off due to being single plates
That engine never was offered as a performance model. RPM is 4800-5200, anything over that power drops off and runs risk of breaking piston skirts as piston is a crossflow type design