I have had the Thunderbird cush drive rubber in my bike for 31,000+ miles after the stock one wore out at 35,000 miles. The Thunderbird{not sport) is a slightly tighter fit than stock.
A cush drive with some play in it is still going to jerk the final drive no matter what oil you're using, so either shim it or get a new one. Excessive cush drive play CAN be hard on your gears. And there is no tolerance limits, there should be zero play.
