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It’s interesting that you ask about what kind of oil. The Johnson T and H models from this time period recommended 40W oil at 16:1. Modern TCW-3 oil is certainly not the same viscosity as 40W oil. I’m thinking that TCW-3 oil is closer to 20W oil.
If the theory is that the heavier oil provides better compression, then does the thinner TCW-3 oil provide less horsepower and does it have a negative effect on achieving a low idle?
I can say from experience that a lower compression HD-25 model that I owned would only idle well with the heavier 40W automotive oil. Other motors with better compression seem to run just fine on TCW-3.
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