Oil, Oil Viscosity and Running Temperatures

[Cunning Plan]

Question.

So, I thought I had oil pretty much worked out, but now I have an oil pressure gauge in the Cherokee, it has got me thinking.

Let's take 15w/40 as an example.

I know that 15 is the winter viscosity of the oil, so 15w is thicker than 5w -right?

I also know that /40 is the running temperature of the oil. So, 40 is thicker than 30.

I know that modern cars are designed with increasingly tighter tolerances, so prefer a thinner oil - 5w/30 or 0w/30 etc.

I know that older engines prefer thicker oils as putting an oil in which is too thin can cause pick-up problems, oil to run past the piston rings and result in burnt oil and general leaks.

BUT, here is what I am now not understanding:

Presuming that - the higher the number, the 'thicker' the oil, or the more viscous it is.

Why does oil pressure decrease as the engine warms?

Why does warming the oil before doing an oil change make it flow better?

The above suggests that the warmer the oil, the THINNER it is, where as the the oil specification states the warmer the oil, the THICKER it is?

[Roadkill]

Quoting: Cunning Plan

I also know that /40 is the running temperature of the oil.

Would that refer to its minimum effective temperature - meaning the oil is working from a lower temperature ?

I remember being told (many times) don't thrash an old car til it's warmed up . . . . I'd guess the /40 means it takes - in this example - an extra 10*C (and the extra time) to get to its minimum operating temperature ?

I suppose it's like washing powder . . . "Works at 30*C !!!" . . . Meaning it does the job at a lower temperature . . . . So "New Bold Ultra" would be /30 whereas "Old Daz" would be /40

Dunno - but tat's how I always thought of it . . . .

[Andy]

The warmer oil gets the thinner it gets.

Come to our factory in the winter, pour out some slide lube oil and it's thick and not really that runny. Come summer with around 25 degree temps, that stuffs all over the place!

I believe it has something to do with the movement of the molecules. Remember heat or temperature is a factor of molecules or atoms bouncing around. I take that to mean that an oil will be far more viscous in this state.

Hotter temperatures will decrease oil pressure because the flow rate is better. Imagine a pipe with a narrow teat on the end, you try shoving golden sururp down there you're going to have a high pressure state as it won't all fit through. Thin that down and it will effectively piss through there giving not a lot of pressure.

This is all my understandings of it. I picked up a bit of knowledge working on one of our machines which kept bringing up a lube system fault on hot sunny days. That year we'd change slide lube oil to some Castrol stuff that was made from rapeseed oil. It was so viscous the pump wasn't able to pressure the line enough to give a positive reading the machine liked.

[Roadkill]

Love the Golden Syrup analogy.

Mmm, Syrup.

[Cunning Plan]

Quoting: Andy

I believe it has something to do with the movement of the molecules. Remember heat or temperature is a factor of molecules or atoms bouncing around. I take that to mean that an oil will be far more viscous in this state.

Hotter temperatures will decrease oil pressure because the flow rate is better. Imagine a pipe with a narrow teat on the end, you try shoving golden sururp down there you're going to have a high pressure state as it won't all fit through. Thin that down and it will effectively piss through there giving not a lot of pressure.

Yes, that is how I have always thought of it too, BUT the viscosity measurement goes from thin = 0 to thick = 1000000 etc.

So why would an oil get 'thicker' as it warms?

15w/40     <  40 is more than 15, so thicker when hot??

[Andy]

http://auto.howstuffworks.com/fuel-efficiency/fuel-consumption/question1641.htm

That explains some of it.

Fancy stuff in the oil, polymers or something that.

[Titsy]

Viscosity is measured in volume that flows per unit time. so a high number means runny and low number means thick...

[Andy]

Not according to the page on that link, the lower the number, the thinner the oil (more runny)

At least that's how I've understood it

[Andy]

Quoting: Linky

The viscosity grade (for example, 5W-30) tells you the oil's thickness, or viscosity. A thin oil has a lower number and flows more easily, while thick oils have a higher number and are more resistant to flow. Water has a very low viscosity -- it is thin and flows easily. Honey has a very high viscosity -- it is thick and gooey.

Ah, see the yanks don't have Golden Syrup so they resort to honey!

[Cunning Plan]

Quoting: Andy

At least that's how I've understood it

That makes it even more confusing.

Engine warms, oil pressure goes down, oil gets thinner, yet viscosity increases?

[Titsy]

Read it again boys High number is thin low number is thick:

Quoting: from your linky

At cold temperatures, the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up, the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. The result is that at 100 degrees C, the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot.

[Andy]

I think I get it, but I don't get it.

[Titsy]

Quoting: Andy

I think I get it, but I don't get it.

Actually, the more I read the more confusing it becomes....

[Andy]

CP... just stick to this.

Left = Bad   Right = Good

Job, Jobbed!

[Cunning Plan]

I also asked the question on VZi and after some fairly similar answers, I got one that made sense:

Quoting: crmalcolm

Your still looking at the cold rating (15W) alongside the hot rating (40).
As you say, the 40 relates to characteristics at 100deg. Now ignore the part after the / and look at the cold rating in isolation.

A 15W oil will start to thicken at higher temperatures than a 5W.

The first part of a multigrade is a rough indication of temperature needed to remain thin enough to use, the second part is an indication of viscosity at operating temperature.

Would it make more sense if you replaced the W ratings with letters.

Eg

0W=A, 5W=B, 10W=D, 15W=D etc.

A/40 oil and D/40 oil are both the same at operating temperature.
B/30 and B/50 are both the same as ambient temperature drops.

A/40 will be less viscous in cold conditions than D/40.
B/50 will be more viscous at operating temperature than B/30.

I can see how the scales can be confused, but on both scales the lower the number the thinner the oil.
Think of temperature, both fahrenheit and centigrade can tell you how hot something is. But 82F is colder than 30C.

I GET it now.

[Andy]

Quoting: Cunning Plan

I GET it now.

Do you?

[Cunning Plan]

Quoting: Andy

Do you?

Yah, do YOU?