Oil Pressure vs Temperature

[ftm1776]

Let me play the devil's advocate......

I've got 300,000 miles on my MK2; very low oil comsumption...
Nevertheless, this is more of a generic question:

With 15W-40 oil the pressure on STARTUP idle is about 70 psi; when FULL HOT(180F), pressure at idle is about 10 psi.

So if the oil acts like 15 when cold and 40 when hot, shouldn't the idle pressure be higher when hot and lower when it is cold??
Seems like it is acting like a 40 when cold and a 15 when hot. Yes?? No??

I know this question makes no sense, but could someone approach their answer like teaching a child?

[RunninWild]

https://www.youtube.com/watch?v=VXJH0xF2UuI

[vanbcguy]

Viscosity decreases when temperature increases.

A straight 40 weight oil would be far thicker when cold than a 15W40 is - in cold weather you'd barely be able to pump a 40 weight oil at all.

Likewise a straight 15 weight oil would be far thinner when hot than a 15W40 is.  Once hot the viscosity of a 15 weight oil would be very very little, it'd almost be like water.

[ftm1776]

Well, it's those little polymers that do the trick....and I thought it was just a bottle of "oil" !!!!!!

Great Youtube video. Clear explanation.

I recommend it to anyone who wants a better ideal of how oil works.

Thanks

https://www.youtube.com/watch?v=VXJH0xF2UuI][url]https://www.youtube.com/watch?v=VXJH0xF2UuI[/url]

[diesel_junkie]

I was also curious about this as well. what is a good oil pressure number while running at normal operating temps?

[Rising]

Someone can correct me if I'm wrong but I beleive the bentley is something like "greater than 28(ish.. Maybe 30?) Psi @ 2000 rpm after the rad fan has cycled at least twice" or something similar to that. They don't give an idle spec if I recall.

Sent from my Nexus 5 using Tapatalk

[vanbcguy]

2.1 bar is the spec I believe @ 80°C oil temperature (which is cooler than rad fan temp)

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[air-cooled or diesel]

I've got 300,000 miles on my MK2; very low oil comsumption...


With 15W-40 oil the pressure on STARTUP idle is about 70 psi; when FULL HOT(180F), pressure at idle is about 10 psi.

low oil consumption is good, 10psi hot, w/15w-40 oil seems some low, i use 5w-30 straight synthetic, i get a fluctuation between 12psi&15psi @head, when hot, the cold # you can kind-of throw out, the hot pressure is more important to know, you could have wear in other places, when a diesel of mine turns 300k +/-, i do engine bearings, and all that goes with it, like seals, water pump, and things, head may have wear too, lowering oil pressure, but i ll say 1 thing you oil pressure (when hot) istn too low (yet), you need to keep an eye on it, or maintenance motor in certain places(main/rod bearings and stuff, etc.).   (?intermediate shaft bearings?)