Thanks for your question.  The clutch in the older thumper may need to be replaced....It has 12K miles on it and the odometer is not working so who knows when it went bad...I  changed the oil and filter on the older bike and used motorcycle specific oil...Don't remember the brand...As said, it is slipping less now than before changing the oil.  Eventually, if I do any work in the clutch, it will initially be just the springs.