2005 Ford Escape Excessive Oil Consumption

Recently purchased this vehicle after having it checked out by my mechanic. On a recent 500 mile round trip the engine started missing under load for awhile, then cleared up and ran fine. On the return trip power seemed low. At the end of the journey I had to stop at a red light. The oil pressure light came on at idle. I eased home the last mile with diesel-like clatter from the valve train at low rpm's. Immediately checked the oil and nothing on the dipstick. Drained oil (about a quart left) and it was black and sludgy. Refilled with Mobil 1 synthetic and filter.
Kept watch on oil and it dropped a quart in 50 miles around town. After research I had the PCV, hose and oil collector replaced. Number 3 cylinder had a very carbony and oily plug. Number 4 cylinder wasn't real good either. Replaced plugs with irridium NGK.
Still using oil and the oil is already darkish. Down a quart in 100 miles around town. Now I've put a quart Marvel Mystery Oil in the crankcase and soaked #'s 3&4 cylinders overnight with MMO thinking stuck rings. Could this be coil related or is it likely rings?