Any high viscosity ( around SAE 80W90 EP) outboard engine gear oil.
According to Evinrude motors from 64 onwards should run on 50:1 but that is an older design without needle rollers in the crankshaft bearings. I think it would be very wise to treat it as pre 64 model and run on 24:1
Currently recommended plugs are J6C gapped at 0.030"