It may just be a lack of previous maintenance (is the oil really nasty when you do change it?). You might want to change the oil more frequently (like after every ride) to get the gear box cleaned up and once that is done (when the oil looks better during the change intervals), it may shift fine for extended periods of time. Obviously make sure the clutch is adjusted properly. Changing the oil every other time is just good maintenance. I don’t always do it, an I don’t always need to, but when I put it off too long, my tranny tells me about it (i.e. harder shifting, up and down).