I installed an oil temp and a water temp guage on my 620, oil hit about 255-260F the other day on a 13 mile tight woods trail, 1st gear stuff all the way. Overheat light was on as well but I think it comes on too low. Water temp guage read about 220-240.
Doesn't seem to hot for oil, especially the mobil1 I'm using, so I dunno if an oil cooler would be helpful, I doub't I'll do so much tight woods regularly.
A water pump that moves water better and extra fan and switch would do more good. I have a fan on the left rad with a manual switch and had it running the whole time.
My overheat light comes on at about 212F according to the temp guage, a little low I think. My water temp sender is in the little hose that loops around direct from thermostat housing to water pump, I may move it to the head and replace the overheat switch with it.
I'd be satisfied if I could get this thing set up well enough to just idle forever without overheating, but it doesn't look like it's possible with just a fan on one side and a pump that doesn't move a lot of water at low rpms.
2004 KTM 625 SXC
2004 Yamaha FZ-1
1996 KTM 620 RXC