Oil Consumption situation

This is a '96 Lumina with a '98 3.1 used engine I installed last year.
I saw the wreck that the engine was pulled from, and the clock had 90,xxx miles on it.
When I first fired up this engine after the install, I kept getting a #6 mis-fire code. I had seen where the last owner had put a new coil for the #6 cylinder within the coil-pack, and a new (different looking) injector on #6.
That tells me he had trouble before he pulled the motor, and did not tell me. Long story short, I replaced the fried exhaust valve, inspected all others, and it runs really well.

The issue I have is that as long as I am driving inside the city (stop and go) my oil level will go down about a quart after about 3K miles.
If I drive on the freeway, I can only get about 500 miles before the "low oil" light comes on. I add oil, go another 500 miles and "Low oil" again.

Is there any educated guess as to where to start to look?

There seems to be clean exhaust, and no external leaking that I can see (No puddles either when parked)

Weak oil and compression rings, bad valve seals, bad or blocked pcv/system. Are the plugs oily? Possible under some conditions the intake gasket could cause this.

Thanks Maxwedge,

Just looking for the most probable to the least probable cause so I can try to find it relatively quickly.

Seems that the higher the sustained RPM's, the faster the oil is consumed.

I will probably address in order:
1) PCV valve
2) Valve seals
3) LIM gasket (Maybe I messed up somewhere when I when I pulled the head to replace the exhaust valve last year)
4) Rings

Note: The plugs appear normal. The cyl. pressure seems to average around 140 PSI on a compression gauge, and all are similar to each other (Thats how I found the bad exhaust valve when #6 showed nothing).

Thanks