They both have the same hot rating(40), so they should have the same pressure. The 5 will flow much better cold.
Those of you running the 5W-40, do you see any pressure difference running warm from the 15W-40?
I have not noticed any difference, but I have the factory oil pressure gauge, so all I can tell for sure is that the needle stays near the O in nOrmal.
I just changed from 15W-40 dino to 5W-40 Mobil Delvac ESP Synthetic (My VW is picky) and noticed:
1) Easier Turning engine when cold starting
2) The Factory Gauge responds quicker when starting the vehicle and waiting for the oil pressure gauge to read pressure
3) My needle also stay in the O on the nOrmal.
4) I have not seen any leaks or oil usage from switching to synthetic/"thinner" oil except my silly plug, I did not replace the nylon washer. It will have to wait.
5) No oil usage whatsoever. (did not use it with the dino either)
6) There is no crossmember to give me fits, or potential fits and a powerstroke oil filter worked perfectly.
I will say that reading over on Bob Is the Oil Guy, not all 40wts are created equal. I am not an expert. But the Mobil 1 European Formula 0W-40 was fine for my early VW TDI's but not for my now picky one. (Engine Code ALH vs BEW/BHW). They mention it is more of a heavy 30wt oil than a true 40wt.