2002 Pontiac Sunfire -Where is my oil going?

I have an '02 sunfire with 129k miles, runs great and is in excellent condition, but in the last year or so I have noticed every time I change the oil (every 3,000m) there's LESS than a quart left in there. It's gotten to the point where I can't get anything on the dipstick after about 1,500m, and I feel like I'm constantly buying oil. No smoke, no smells, and 3 people checked and can't find a leak. Starts running a little rough after about 1,000m after a change. I know it's got a lot of miles on it, but this seems excessive. Is this normal for an older model? I heard checking/changing the valve gasket might help, but can't find any info on it. And I switched to thicker oil last year when it started. Any suggestions would be much appreciated, not ready to give this car up yet!