His shop foreman suggested a qt. of MC 5W30 synthetic. He has a few qts. in his station and was going to give one of them to my son.
I agree with the other posters, I would use Amsoil if possible. I assumed you meant what to use if he had no Amsoil on hand. Any oil is going to be better than running 1+quarts low.
But since he has Amsoil in there and its an extended drain oil, I'd either use Amsoil, or if thats not available I'd use another extended drain oil like Mobil 1 EP. Especially if he is going to go another 7k. I also agree with others, get a uoa at 7-10k before going blindly to 15k.
Before just going 15k i'd also look at driving habits, if its mostly highway runs, thats one thing. Short trips and a lot of city driving? I'd think twice, especially in cold weather.
I agree also with 4WD, what filter is he running? Motorcraft? If so i'd run an extended drain filter like Fram Ultra, Toughguard, or an Amsoil filter.