I wonder where the oil is going - if your mechanic is correct, and there are no oil leaks in the block (which, not to undermine him, but nearly ALL Alfa's leak), then we need to look at other possibilities. When you park the car next time, put a clean piece of cardboard or paper underneath the entire engine, and let it sit for a while. Even a small leak, when the oil is warm, could realllly be spitting out a lot of oil at highway speed.
The next question is if the car smokes - a blue oily smoke on hard, full throttle acceleration - or blue smoke when you snap the throttle closed after hard acceleration. Either of these will indicate worn rings or valves.
My best guess is that you have a tiny weep somewhere, and also, you're using a thinner grade of syn oil that may not fare well in stopping oil burn off.
So, again, I would switch to 20w50 dino of a good brand, and then drive that sucker around some more...take the GWB to 80, all the way out to Sussex County NJ, or even to the Delaware water gap, and drive up along the river to Port Jervis, then take 23 South back down to 80. Open it up some!
And top down too please!