2008 Pontiac Grand Prix - no heat (cold air) and high temperature

No, nothing about adding oil would affect the cooling system like this, unless you manhandled the engine wire harness and affected the engine cooling temp sensor(s). Try checking your coolant level, hose connections, and radiator cap. Make sure the radiator cap is making a good seal, and the hose to the overflow is intact, a vacuum leak there would prevent proper coolant flow. If you do decide to replace thermostat, watch a YT video or two, it's more than doable for a DIYer. Only need basic tools. Any time you replace an item that has a gasket, like the thermostat housing, be sure all the old gasket material is scraped completely off, so the new one makes a good seal, and always check the thermostat is put in facing the correct direction. After installing new thermostat, and adding coolant, you'll need to make sure all air is bled out of system, on newer cars that's fairly easy though.

Sounds like you're very low on coolant! Hope that helps! Jim