24:1 for the 18hp
Exakery! The 5.5 and 7.5 horse motors have bronze sleeve type (friction) bearings on the crank and rods which need the extra oil while the ten horse use a combination of friction and needle/roller bearings while the eighteen horse and larger used needles and rollers throughout. Using a good brand of TCW III rated marine oil at the proper ratio instead of the motor oil suggested back in the fifties will help ensure the motor will still be running for your grandkids to enjoy!
Can’t say it any better!
PLEASE ignore all the advice about using less oil on these engines! Yes, today’s oils are better, but not enough to allow using anything less that 16:1 on your bushing engine! A well tuned/adjusted engine will not smoke much at all, even at 16:1!
Enjoy your beautiful 1958 Johnson, please check change the gear lube before running the engine. You should also consider replacing the water pump impeller unless the seller can confirm it has been replaced recently.