I always have used valvoline racing oil 20W50. My oil is changed once per year with about 100-300 miles on it. It comes out almost as clean as it was from the bottle.
Darker colored oil indicates some cleaning was going on, although if it becomes dark too soon i would look for any source of outside contamination coming into the engine via the valve cover breather cap. Or maybe your carb is running overly rich. Just my
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