Officially Ford advised 10W30.
The difference between 10w30 and 10w40 is the thickness of the oil at operating/high temperatures, at which 10w40 is thicker than 10w30. Using 10w30 oil in cold weather will help reduce excessive oil temperatures and drag as the engine warms up. Using 10w40 oil in the summer will help the oil stick to internal components in high temperatures, avoiding the wear and tear from metal-to-metal contact between moving parts.
The only difference between 10w30 and 10w40 oil is their thickness at engine operating (hot) temperatures. They will flow at the same rate during a cold startup since the starting viscosity is “10” for both formulations.
I used 10W40 without problems (with the oil/water cooler in place and a decent radiator).