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[Particle Man]

Originally Posted by Homeslice

I'm guessing the only reason Harley calls for thick oil is because their engine tolerances aren't very tight

Originally Posted by Gas Man

It could be the air cooled.

I'm with Gas on this - if it were an older Harley I'd say it was due to the tolerances not being very tight (the ol' leaking oil all over story). These days, they seem to be better able to hold their oil; I'd think you would want the heavier oil for the air-cooled engine to prevent hot-spots caused by oil getting heated and being too thin in spots.