The 5 1/2 hp is a gem. Runs forever, putt putts at idle very smoothly. Had one over 40 years ago. It was used and abused. Overheated severely, submerged it when I flipped flipped the boat, ran it for several days with no oil in the gearcase. Water cooled/lubed gears don't have a long life, neither do the propshafts. It ran from 1964 till we sold it in 1990. Only repairs were water pumps, shift lever, all gearcase internals except for driveshaft. Never touched the power head even after it got hot.
Pre 1955 was pretty much 16:1 gas-oil, most mid 50s thru 1963 ran 24:1 oil mix, some of the 54 - 55s may still have run 16:1. In 1964 OMC switched to 50:1 for the full line. Some engines like the earlier mentioned 5 1/2 weren't completely antifriction bearing motors. The lower crankcase bearing was still a bushing. However even at 50:1 there was enough puddling of the oil to keep it well lubed.
Here's where some disagreements happen... Some of the earlier engines, the 1957 -1959 35 hp were fully antifriction bearinged and could run 50:1. The 1960 - 1963 40hp models were identical bearingwise and clearancewise as the 1964 model and can use 50:1 also. Same can be said for the 1958 - 1963 V4 models. In the 1970s OMC did issue a service bulletin saying their OMC, Evinrude, and Johnson Oils could be run at 50:1 in all of their engines. Myself unless the engine was completely antifriction bearinged, late model 5 1/2 excluded, I wouldn't use 50:1 but the richer 24:1 or 16:1.
The mid 1980s 100:1 issue. Some engines did die younger during this period, but not because of lack of oil while running. If used very regularly and the oil was properly mixed with the gas, they showed no signs of lack of lubrication. It was when they would sit for longer periods of time, especially when owners didn't fog the engine for winter storage. The 100:1 mix just didn't leave enough residual oil on the crankcase bearings and rust formed on the journals.
The 18/20 hp was a classic too, was marketed till the mid 70s. in 1969 - 1977 a hopped up version became a 25 hp