What oil should I put in this truck?

I have a 1987 Ford F150 with the inline 6. My part store has a special on Mobil 1, where you can get the 5qt jug of 5w30 or 10w30 full synthetic and a Mobil 1 oil filter for $22.00. If I buy the Castrol GTX I normally put in it, after buying a 5qt jug of that and a oil filter it runs about the same price. What should I use?