I also agree considering the age and mileage of most fox's these days. 10w-30 seems to be the all around best oil. But obviously if you are in a climate that tends to be a little extreme in one temperature or the other then another weight may be better suited for your stang. I am a Amsoil dealer and after having delt with them for sometime I have also begun to realize that the weight recommended also seems to be based on the oil company. For instance Amsoil tends to recommend a lighter weight as compared to other manufacturers, I believe this do it part to all their oil being synthetic, but idk. H*ll they actually recommended 0w-30 or 5W-30 for my 94 cobra. Not that they new this but my car only has 15,000 miles on it too. So it's a little "tighter" as compared to a higher mileage car. But I went with 5W-30.... I don't care what they say its not a import motor and the tolerances aren't going to be as tight as a newer motot. Also, they did not know this either but my Cobra only see's pavement during the summer months soo. I thought it would be best for my car and its purposes.
I either use Amsoil full synthetic of course, since this is all they produce! Or Mobil 1 clean 5,000 10W-30 in my other toys... But I'm sure someone else thinks differently, Opinions are like *ssholes everyone has one! lol
Good luck though