Engine oil- usually breaks down quickly under the stresses of wet-clutches and doesn't have the required properties to support wet clutches. It will work for a little while, but may break down sooner and glaze your clutch material.
MC/ATV oil- engine oil designed to support/withstand wet clutch and high pressures that gears produce. A typical 10w-30 is lighter than gear oil (less drag, less power loss)
ATF- designed for automotive automatic transmissions. Does well if kept under 200 degrees (think auto trans cooler). If your banshee sees short runs where it is allowed to cool down you can run this. Even lighter so it has the least drag and power loss. However, ATF has personally costed me a trip. After 6 hours of hard riding, my clutch glazed over/slipped and the oil reeked of foul burnt ATF.
Gear oil- Offers the most protection but is also the heaviest. More drag, more power loss.
Note- When I claim less/more power loss.... I'm talking so minimal you probably wouldn't even notice on the butt dyno.