An additive probably won't fix your problem. My 98' was burning a quart every 600 miles due to the oil control rings being stuck into the piston. Last winter, I pulled the head, sent the head out for a valve job, and re-ringed the engine. It cost meabout $700 in parts and machine shop charges.
Do a dry compression test, then a wet compression test. Even though I had issues with my oil control rings, I still showed 190 across the board on a dry test. After adding a little oil to each cylinder and rerunning the test, the numbers shot way up leading me to diagnose the ring problem.
Here's a picture of what the rings look like when you have this issue - not pretty:
http://i65.photobucket.com/albums/h2.../engine017.jpg
