Welcome to the forum!!!
I use 15W40 in a number of vehicles I have and have been very pleased with the results.
I noticed however, that you live in Wyoming. (My wife is from Wyoming - a little town called "Manville"). As I recall, it gets pretty cold out there so you might have some starting problems when the weather turns sub-zero.
I run 4Qts of 15w40 in my 73 Chevy and 1 Qt of High mileage just fine. I don't drive in the winter, is a sports car not a truck.
15w40 is a good oil for older cars/trucks. QS 10w30 high mileage is also a older style oil with higher Zinc than current auto oils that would work well if it gets colder where you live.