I have a 2008 Ram 1500 SLT with the 5.7 with 198k miles. Recently had to add the wrong weight oil in an emergency but then changed it out with the right weight Mobil 1 (which I always use). Also removed throttle body and cleaned it off the engine. Since then, the engine will start and run fine in the morning but in the heat of the day it either wont start or will start and then choke and die. No CEL but pending codes of P0171 and P0174 (system too lean bank 1 & 2.)

 

Check fuel pressure.

 

I checked and no fuel pressure at the rails. Now I have to figure out why there is in the morning but not in the afternoon.

 

Does the pump prime when it's hot out? Might try thumping on the bottom of the tank when it doesn't wanna start, see if it changes anything.

 

I would just change the fuel pump now. But if you want to do more checks; next time the truck won't start turn the key to run, then go tap on the fuel pump and see if you can hear it prime. You'll probably hear the pump prime after thumping it a while. Then the truck should start.
I don't recommend spending a lot of time troubleshooting an erratic pump.