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Castrol GTX 5W-20. But the dealer uses Motorcraft 5W-20 which is a synthetic blend. Supposedly I don't need to change my oil but every 5,000 miles (but I am not sure I agree with that).
I used Penzoil 5w-20 early on but found that Castrol leaves less soot crap on the pipe tips, so I have been a Castrol user for 7k miles and like it a lot.