I haven't put much time in on Sportsters to give you a knowledgeable reply there from experience but most better gear lubes should do a good job in there. Spectro the makers of the oil that I use also have another gear lube in the same range that doesn't specify 6 speeds, not that I feel the 6 speed Spectro wouldn't do a good job. Mainly use a gear lube and not an engine oil of your choice.