Use to be that the 15-50 red cap was fine....now they've switched the formulation, honestly, I don't know.
I run the Amsoil MCF 10W-40, paid around 7.50 / quart shipped to my home.
The mobile MX4T is a good oil, but as you've said, its kinda pricey.
If you're looking to save money, consider the Shell Rotella 5W-40. Cost's less than $15 / gallon. It's a pretty good oil classified as a "Synthetic", but really it is a highly refined conventional oil. Still pretty good though. It wont last as long as the Amsoil or Mobile true synthetics.
IMO - those are the top choices.