Cowboy, your mileage is pretty typical (members claim to get between 13 and 18 mpg consistently). Your oil consumption rate is high, but not uncommon.

Oil often gets past poorly-seated, old, worn piston rings and burns up in the cylinder very, very slowly - think like a microliter per combustion. You'll never see white smoke in your exhaust at that rate, but that's probably where it's going. Replacing the rings means cracking the engine open and pulling the pistons, and there really isn't a much bigger (and expen$ive) project than that.

You can try the seafoam treatment, replace your PCV valve, and clean the EGR valve and throttle body - these may help, but at 100k, my money is on the rings as the culprit. Just keep adding oil - it's a lot cheaper!

Good luck!