But thinner oil runs cooler than thicker oil (less friction) - so you have a wash there, Gordo. You are about 20 years behind in your understanding of oil and engine technology. 10W-30 and 10W-40 is for old school pushrod motors with a lot sloppier clearances than the modern high tech OHC engines.
Use the recommended weight oil. Doesn't matter what brand and type you use as long as it meets Ford 930 specs. However, the Motorcraft semi-syn is the most cost-effective oil to use, all other things being equal. There is no need to use a full synthetic unless you want to extend your drain intervals. The fact that the Motorcraft oil is so cheap at Walmart makes it almost a no-brainer.
Oh yeah - Gordo, Ford does not make Motorcraft oil. Last I knew, Conoco makes it.