Both 10w-40 and 40w should be the same vis at 212f. I'll bet your oil is gettig hotter than 212f to show this. At 250f the 40w will show better pressure than the 10w-40. Someone here will say to use 15w-40. I'd use the 40w or a 20w-50.
i used 10w-40 in my 74 nova for a while and kept getting low oil pressures at idle after the engine warmed up. i also kept getting metallic particles on my magnetic oil drain plug (its a 350 with 10k on it now). i switched to straight 40 weight and the oil pressure is substaintially better at idle and more consistent throughout the rpm range. i haven't pulled the drain plug yet, but i have a feeling its not going to look as bad.
am i mistaken, or do i just not need a multi-weight oil in houston? it never freezes here anymore, and it gets pretty warm. would my fram oil filter have anything to do with this (which i'm not using anymore, since i noticed the filter element had come loose inside!)?
pressure is not as important as flow. the higher the pressure, the lower the flow. So once you get enough pressure, you don't want more. If you are not getting enough pressure with a 15w40, there is something wrong with the pump or your pressure guage.
Almost no manufacturer recommends a straight 40wt oil, and those that do allow for it do it when minimum temps stay over 60 or so.