What make of outboard do you have if it’s older than 1920?
The older motors didn’t have seals on the crankshaft so the oil mixed with the gas was relied on to help seal the crankcase. Johnsons in the 20’s and 30’s called for 40 or 50 weight oil in ratios up to 4:1 for some of their racing models. Aviation oils designed for air cooled piston engines in W80 or W100 (SAE 40, SAE 50) are good substitutes for the older oils.
https://www.shell.com/business-customers/aviation/aeroshell/piston-engine-oil/w80-w100-w120.html
