in my own truck I use Mobil 1540 from costco since its a very good price. i still get the filter from walmart, and get the air and fuel from DIS or other online sources.
in the company trucks I use napa's oil. sometimes shop around, and sometimes bite the bullet and buy from NAPA or ford.
as long as you keep the oil in there, the psd will keep ticking. I have a truck 12 years old in the fleet and its still cruising along.