I've just changed my oil and originally put diesel oil into my 2000 Pontiac Grand Prix accidentally, but removed it before ever turning it on and put the correct oil in. Now 1000 miles later, it sits at a higher temp than normal but only when idling or driving rather slow. It smells like burning oil sometimes, but it's not low or anything. I don't know what's going on.
Slight overtemp corrected by moving the vehicle suggests to me that your cooling system is not operating properly and that it needs the extra cooling coming from moving air. Inspect the following:
- coolant level: extreme low coolant can introduce air into the system and compromise cooling.
- radiator cap: the cooling system needs to be pressurized in order to prevent the coolant from boiling. Rad caps are about $10, test yours, you may want to replace it if it's suspect. Only remove it when the engine is cool, otherwise the whole works may literally blow up in your face.
- thermostat: these wear out, but are usually inexpensive and easy to replace. You will need to drain the coolant to have access, so you may as well do this in conjunction with something else that requires draining the coolant anyways.
- water pump: this moves the coolant around the system. With the engine off, make sure the belt is tight and that there is no coolant splashed onto nearby parts. A leaky water pump needs to be replaced. This is one of those things that needs the coolant drained for access.
- radiator flow: with age, the cooling system accumulates all kinds of crud and internal deposits, especially vehicles using DEX-COOL (as I believe yours is). Crud and deposits will restrict coolant flow, and that will restrict cooling. Drain your coolant, fill-up the system with a cleaning product that removes calcium deposits, lime and rust, and run the engine to temperature. Once the engine is at temperature, turn on the heater set to max heat and keep running the engine for a few minutes. Then drain the system again and fill it up with new coolant.