I would run whatever weight the OM calls for if it was a new or rebuilt engine.
BUT
In a high mileage engine I would run what ever weight required to get the correct oil pressure at operating temperature.
If you get the correct pressure at 190 degrees F on 10W40 it would be good. Use a good mechanical gauge for testing the ford dash gauges are for show only!
The ford bb engine requires good oil pressure to keep the mains lubed properly. Before I went through the last rebuild on our 460 I was using straight 30 diesel tractor oil to keep it running without knocking.
Believe it or not, straight 30 holds more pressure at 190F than 15W40 on a given engine.
The 2 stroke detroit diesels require straight 40 weight so it is available if the bearings are really a mess.

I run 15w40 diesel oil in our 460 and Mopar 440 both engines are doing fine.
I am in DALLAS and the temp rarely drops below 30F, its the summer that a bear at 105 for weeks on end. 5W30 at that temp is VERY thin at temp.
I also ran it when I lived in Napa Ca.