OIl Spec.....Hey What Gives????

[ftm1776]

I'm using Delo 400LE 5W-40 synthetic these days.......
Also, watching a recently installed oil pressure gauge.....this question arose......
On start-up, i.e. cold, the pressure at idle is around 65 psi....
After the engine is fully heated after running, the pressure at idle is 10 psi....

If the oil spec is "5" when cold and "40" when hot, shouldn't the pressure at idle be lower when cold and higher when hot?

This is kind of a "devil's advocate" question, so humor me and straighten out my thinking on this.

Disclaimer: I am not a lubrication engineer and I haven't talked to any Chevron reps.

[theman53]

it is not 5 psi and 40 psi. It is the multigrade viscosity of the oil. It acts like 5weight oil when cold and 40 weight oil when warm.

[RunninWild]

Cold oil is thicker then it is when its warm hense the higher pressure. Cold doesnt necessarily mean air temperature when they weight the oil. 5w40, would mean its a 5 weight at a certain temperature, and then its 40 weight when fully warmed up. When the oil is colder (around 0 or below) it is actually a very high weight, as it warms up it drops to the cold rating, then when fully warm the molecules in the oil expand and increase the weight. There's a lot you can read on the subject, I've forgotten 90% of what I learned about oil  in school but I'm pretty sure thats the gist of it...