I use Pennzoil conventional in my 03 Park Avenue with the same engine, only supercharged. Car burns a bit of oil, not much, but I don't wanna have to keep adding expensive oil each time it needs it. 144k on it, and best road car I've ever owned.
On the DexCool issue, it's known to eat through the intake gaskets if not changed often enough. Change it every 3 years instead of the recommended 5 and it shouldn't do it. Problem with these engines is the intake manifold and EGR stovepipes are made of plastic and they usually start leaking around the stovepipe and it can cause the engine to hydrolock and seize it up. You can replace them with improved aftermarket gaskets made of aluminum, also a aluminum stovepipe, which is what I did for safe measure, mine weren't leaking, but I wasn't going to wait for them to start either! Other than that, the 3800 will run until the end of time.
Also keep an eye on the transmission fluid and change it often, the ATX is a weak point in this era GM FWD car but usually the problems are caused by neglect more often than not.
On the DexCool issue, it's known to eat through the intake gaskets if not changed often enough. Change it every 3 years instead of the recommended 5 and it shouldn't do it. Problem with these engines is the intake manifold and EGR stovepipes are made of plastic and they usually start leaking around the stovepipe and it can cause the engine to hydrolock and seize it up. You can replace them with improved aftermarket gaskets made of aluminum, also a aluminum stovepipe, which is what I did for safe measure, mine weren't leaking, but I wasn't going to wait for them to start either! Other than that, the 3800 will run until the end of time.
Also keep an eye on the transmission fluid and change it often, the ATX is a weak point in this era GM FWD car but usually the problems are caused by neglect more often than not.