Welcome, johnd. When the bike is on its kickstand, do you see any bubble at all in the top of the sight glass? I'm inclined to say drain out some oil. The drain plug is at the bottom of the engine, but there's another bolt down therethat sometimes is mistaken for the drain plug. You can download the service manual from this website and learn more there. Check the link.
http://www.gz250bike.com/forum/viewtopic.php?f=7&t=2158