[s-cars] Thermostat woes

[Aaron]

Hi all,
It appears I have thermostat issue. Can you guys confirm -
Water temp gauge functions but only reaches about 1/8" above cold (maybe
higher if its a warm day). Fluctuates somewhat depending on outside temp.
Oil temp stays low and does not reach the first tick.
Heater works but ability to blow hot air is less after a long downhill
as motor temp decreases.
Are there any issues to be aware when replacing the thermostat?
Thanks in advance
Aaron