thermostat issues?

i drive a ton of highway miles in my oil burning 1990 Galant ES. the temp gauges sits at cold on my 2-3 hour commute, but running the snot out of it around town or pulling over to the side of the road and turning the car off temporarily and starting the car back up again "wakes up" the temp gauge and it will move, but once on the highway again, it'll find it's way back to the cold side of cold, lol, and i, i will be freezing for no hot air blows.

so, basically i need a thermostat right? if i do, i have choices between a range of 160 degree to 180 degree and i have no idea which is right. heck, i don't know if it's really the thermostat but it sounds like it.

any suggestions to what it may be if not the thermostat and if so, what's the OEM degree thermostat i need, or will it be better to get a thermostat of a different temperature rating?

thanks in advance!

EDIT/ADDITION:

Haynes is telling me to drain the entirety of the cooling system to get this job done, is this needed? May I just yank out the old thermostat and sop up the overflow and then replace it? Also, some places tell me to use silicon as a seal for the housing, some say RTV sealant on boths sides of the gasket.

easy peasy. drained one gallon from radiator, which has the 3/8 inch tube built in by the way. then i removed the upper hose from the radiator because the hose is corroded onto the thermostat housing. unbolted the thermostat housing upper hose and all, popped out old thermostat, had to chip off some old gasket, installed everything and topped off the coolant. now i have heat, whoot!