Hi Mumbles – Your ’42 chart was upgraded in the 1950s. Here in one from ’47 that hasn’t been upgraded.
In the 1920s and early ’30s oil refining was more art than science. Quality seemed to depend on the phase of the moon. Evinrude did extensive testing (Everything on the motor was supposed to last at least 1000 hours.) Whenever a perceived failure in lubrication happened, the tendency was to add more oil to the gas. The recommended mix for the Racing 460 was a quart to the gallon. (Yes – one gallon of oil to four gallons of gas)
Government funded research during WW2 brought vast improvements in manufacturing and chemistry. Having to meet government specifications brought more uniformity to the oil industry. Further testing proved that much of the piston scoring thought to be from a lack of lubrication, was from carbon deposits from excessive oil flaking off combustion chambers and getting caught between the piston and cylinder wall. This resulted in less oil being recommended. Lubrication charts from the mid 1950s reflect these changes. . . . . . 😀
