I own a 2004 Buick Park avenue with 145,000 miles on it. I change the oil every 5,000 miles, and I have on it a high flow catalytic converter from ZZP and a flowmaster super 44. Recently, i have noticed my "add engine oil" light comes on every 600-1500 miles or so and I have to add a full quart of oil every time. Within the last month I have added over a quart. There is nothing leaking from the car, and there is no smoke coming from the exhaust. Is this normal for a car to use this much oil, or is something wrong?

No it is not normal....... if definitely sure there is no external leak(usual spots are the oil pan, valve covers, and oil pressure sender), then I would be checking the valve stem seals......they usually have a metal ring around them......if missing that is your problem......

Agree, and definitely the oil pressure sender. Not much gets on the car from that going bad, especially if it doesn't spend much time idling.

Also check the pcv valve and o-rings. Those can also cause all kinds of fun issues when they fail.

What brand and weight of oil are you using?

I'm using 5w 30, i used to use the penzoil synthetic but recently this most recent oil addition I went to the dealership itself and they used whatever GM oil they put in their cars.

Are any of these spots that you have asked me to check easy to get to or do I need a professional to look them over?

Most listed by Tech II are easily checked, but require access to the underside of the car. The pcv valve can be checked easily up top. You can usually tell if that is a problem by seeing if the car changes the way it'* running by taking off the oil cap at idle. No change probably means no pcv problem.

Also, be careful with "whatever oil they have", if it isn't 5w30 or 10w30, the engine is probably not going to be happy. I learned through "bulk oil 5w20" experience that 3800'* don't like 5w20.