You should have asked why he thought the 0W-40 was too light. True, M1 0W-40 is a light 40 weight (if comparing to Rotella 5W-40 or Red Line 0W-40) but that isn't something that is known by looking at the bottle or because the first number is 0. Go back into the store and tell him that the kinematic viscosity is 13.5 at 100 degrees Celsius, and the High Temp High Shear is 3.8 centipose at 350 degrees Celsius, and that it meets European ACEA A3 requirements for shear stability, so it should handle the Texas heat fine. Or you could just tell him to pound sand.
He probably also thinks that 0W-30 is too light while Syntec 0W-30 is one of the thickest 30 weights you can buy.