One quart of oil burned every 5000k (3106 mi) is not too bad at all. Even two quarts in that distance is no disaster. But seeing your fuel consumption go from 21.4 mpg to 16.8 mpg is a sure sign that something is wrong. With 152,xxx miles on the odometer and an original O2 sensor perhaps a new 02 sensor is in order.

I think a quick test for excessive blow-by can be made by taking your oil filler cap off while the engine is running. If you can place the cap on the filler opening and let it rest there, with the engine running, but not screwed in and it doesn't blow off you can figure that blow-by is not the problem.

When a PCV valve has been in a 22RE valve cover for 10 - 15 years the rubber bushing/seal becomes as hard as a rock. maybe they don't seal the oil well when they are like that? It sure makes it hard to pull the PCV valve out too. Most auto parts stores have a replacement for the PCV rubber bushing/seal. -- Matt