Low oil will definitely heat up the engine, from all the friction that is going on that isn't supposed to be going on. I have a 2000 SLS Seville with 100k and it uses a lot of 5w30. I switched to 10w40 and it helped with consumption a lot.
The Northstar V8 is a great engine, but the STS was a pretty hot sedan back in the day and the original owner might have had a lead foot as well. If you didn't own it when it hit around 70k, you probably can assume it's been run since then chronically low on oil if the previous owner didn't pay close attention.
That said, 3 to 4 quarts down on oil is either a leak you would see in the form of a puddle where you park it, or a cloud of blue smoke when you start it or stomp on the gas. If its not a leak, you have a seriously worn engine. You could get a compression test or some other diagnostics for advanced engine wear if you want to consider doing something about the problem, but as another post said, it might be time to sell it for parts and buy a new ride.