The 1991 factory owners manual shows 10W-30 for temperatures above 0F. That is not an opinion, just Ford's recommendation. That temperature range would cover all of North America except northern climates during the winter months. I live in a temperate climate in Dallas and have always used 10W-30 to the engine's long-lived happiness.
The 10W-40 grade was discontinued by some brands because of the premature deterioration of the viscosity index improvers (VIIs) that were required to attain that extended viscosity range. I would think that newer oil grades have limited this problem, but the 20-year old Ford SHO manual does not include the 10W-40 range.