Good points posted by "67hat34c" about the use of ATF in a sportster trans/primary. The same would hold true for the primary in big twins too.
Automatic transmissions need this thin fluid to operate the high pressure hydraulic systems. They do have gears and some like the Honda transmissions closely resemble a manual transmission. However the oil is under pressure and coating the gears under pressure and high flow. In your sportster transmission the gears sit in a little oil and it climbs and splashes. ATF will not cling and climb as needed etc. Would also not use it in a Big Twin primary for the same reasons.
Automatic transmissions need this thin fluid to operate the high pressure hydraulic systems. They do have gears and some like the Honda transmissions closely resemble a manual transmission. However the oil is under pressure and coating the gears under pressure and high flow. In your sportster transmission the gears sit in a little oil and it climbs and splashes. ATF will not cling and climb as needed etc. Would also not use it in a Big Twin primary for the same reasons.
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