How many miles/hours are on the engine? the 305 is a heavy duty industrial type engine, and it's probably got a little wider clearances than most similar-sized automotive engines. I'd recommend something like high detergent straight 30 weight DELO. If very low winter temperatures are encountered drop back to 20W-20 during the winter. That's what was recommended for most commercial engines from the 1940's and 50's.
Jerry

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