When I had my GT I ran several different oils- Ford semi-synth, Pennzoil and Quaker State hi-mileage, but I ALWAYS ran the Motorcraft filter. The drainback valve is a must to get oil up top when you start it! I ran both 5W30 and 10W30... my timing chain guides had seen better days and it was using a quart of oil every 1000 miles or so.