I've got a '95 Ski-Doo Grand Touring 670 I've had for a year now. The temp gauge has gone up higher than I like (over 3/4 to the top, wish it had real numbers) so I thought I'd siphon the gross oily coolant out of the reservoir and put in some fresh.

The stuff I pulled out looked odd, its really thick and REALLY green, like straight coolant... I put the tester in it and if its normal coolant its plenty anti-freeze (like good to -40F)...

I put in some 50/50 Zerex G05 which is really good coolant I used in my Mercedes cars and is supposed to blend with all other coolant types. As a test I poured some of the green stuff into a cup with some of the G05, they didn't seem to react to each other so I'm not super worried. At some point I plan to pull all the coolant out and replace with fresh, I know that coolant loses its anti-corrosion properties over time.

What do you guys use and how often do you do it?

I usually have mine tested by a buddy who has a tester of some sort. I do change mine about every 4 years. i would put a new thermostat in it if you are worried. To check it find a heat temp gun and shoot it with that to see what it really is. Shoot right next to the sensor. That will eliminate wether it is a thermostat or a gauge problem. Usually those gauges are pretty accurate. I am not %100 positive but I believe that the thermostat is a bitch to change on that. If King is on here he will tell you exactly what you need to do.

My 96 F3 600 had an overheating problem. Turned out to be the head gaskets (O-rings). They looked perfect, but apparently there was an issue. Yours may be starting to go. I pounded my head against the wall for a few weeks trying everything known to man until I finally just bought the gasket set and swapped them out. Coolant is coolant as far as I'm concerned. Hell, I've used Gatorade/water to get by in a pinch, but it wasn't a sled. Believe it or not, Gatorade will keep water from freezing to a certain temp.

Good thinking I've got one of those infrared thermometers.

I don't think its actually overheating it just seems like its hotter than it ought to be but I don't really have any idea what it is since the stupid gauge isn't labeled. I'll play with it and see if I can get real numbers.

Is it super ghetto to write the real numbers on the gauge in sharpie?

Mine would boil over (literally) at about the 3/4 mark on the gauge. It was blowing exhaust gas into the cooling system.

That makes some sense, the system couldn't pressurize so it would boil... Mine isn't actually overheating that I can tell.

I think I may have a sticky thermostat but the new coolant seems to help. I rode for an hour tonight and after about 45 minutes the temp gauge started riding more in the middle of the range.
I should do a thermostat I guess.

Take it out completely on a test run. Let it warm up good and ride in good cooling conditions that will answer your question.

I think the new coolant has fixed it or at least a big improvement... My guess, and its only a guess, is that the guy who had the sled before me had used straight coolant with no water. From what I understand straight coolant doesn't freeze but also doesn't transport heat very well. Its also possible it was sticking in the termostat some as it was quite thick stuff... Adding some proper 50/50 coolant/water has thinned it out some.
In the spring I'll drain all the coolant out and put in a proper 50/50 mix.