Oil Viscosity

[SamsBus2012]

Coud someone please clarify how oil grading actually works (in english please, short words and no redirects to technical websites) as I am a bit confused - quite easily done as my grey matter rapidly degrades to the consistency of soup!
So, with the oil I install in the van, 15 w 40, the 15 refers to the viscosity when cold and the 40 when hot?? Right so far? And the W refers to "winter" additives?
Now my undersatnding is that the higher the number the greater the viscosity so 15w40 is more viscous than 5w40. So how does the 40 rating work becasue when my oil is hot its as runny as heck!
I'm obviously missing something that my 5 year old son would spot so could the great and enlightened on this forum please clarify?

[R0B]

http://www.driverstechnology.co.uk/oils.htm" onclick="window.open(this.href);return false;

[SamsBus2012]

Thank you Rob. Still none the wiser really. I can only assume therefore, that the prefix number must be on a different scale or units to the suffix number otherwise surely, on the reasoning that the lower the number the thinner the oil, when cold (as in the example of 15w40 for instance) the oil would be thinner than hot which clearly its not. I'll just stick to what I know and

[bigherb]

Mono grade oils steadily get thinner the hotter they get.
A SAE 20 oil will get very thin at 100 deg C (say it becomes like water)
A SAE 50 oil is thicker to start with and won't get so thin at 100 deg C (like milk)
A multigrade 20W50 oil it starts of as a SAE20 oil but as it gets hotter it doesn't thin so much as a mono grade oil and at 100 deg C it is only as thin as a SAE 50 oil would be.

[CovKid]

I can't for the life of me figure out why some v-dub owners are so fixated about bunging in monograde but I still bump into them from time to time. You can't shake them.

I suppose its akin to saying: "Bread and dripping was good enough for my Dad so its good enough for me" - despite the fact that his Dad keeled over from a heart attack, and as a substitute for butter, dripping is terrible in jam sandwiches.

[nicktennear]

So Covkid, is it alright to use dripping from Eurocarparts or should I get dripping direct from VW?

[AdrianC]

So, with the oil I install in the van, 15 w 40, the 15 refers to the viscosity when cold and the 40 when hot?? Right so far? And the W refers to "winter" additives?
Now my undersatnding is that the higher the number the greater the viscosity so 15w40 is more viscous than 5w40. So how does the 40 rating work becasue when my oil is hot its as runny as heck!

Back in the day, oil was "monograde". It thinned as it warmed up, in a linear way. A 40 weight oil was a certain thickness cold (0degC) and a certain thickness hot (100degC). Same for a 20 weight oil, which was thinner than the 40.

A 20w40 behaves, at 0degC, like a 20 weight oil at 0degC. At 100degC, though, it behaves like a 40 weight oil at 100degC. So, cold, it's thinner than a straight 40, but hot it's thicker than a straight 20. Obviously, though, it's runnier hot than cold. That's a basic laws of physics, which applies to oil just the same as golden syrup.

A thick cold oil takes a lot of pumping round the engine, sapping power, and providing VERY high oil pressures, which can blow seals.
A thin hot oil gives lower oil pressure, especially in a worn engine with bigger gaps.

[Timburrows]

In the john deere tractor manual 15 w 40 oil is described as minus 15c to plus 40c centigrade temperature range and like wise 5w 30 would be -5 to+ 30 but that was 20 years ago so it probably changed now

[AdrianC]

In the john deere tractor manual 15 w 40 oil is described as minus 15c to plus 40c centigrade temperature range and like wise 5w 30 would be -5 to+ 30 but that was 20 years ago so it probably changed now

That's pure coincidence, not least because both John Deere and the API (who set the viscosity ratings) are American, so wouldn't know what a Centigrade was if it bit 'em.

[Timburrows]

Cent one hundredth of a dollar, grade something to do with school!!! No that's what was written it the manual but I know what you mean

[AdrianC]

that's what was written it the manual

(after translation into English)

[SamsBus2012]

Thanks everyone. The grey fog is lifting and I now understand (i think).
Anyhoo.... van has 15w40 mineral as recommended so I guess that is what really matters.

[AdrianC]

van has 15w40 mineral as recommended

VW's recommendations aren't so simple as that...
http://www.westfaliat3.info/VW_T25_Inst ... iaT3WM.pdf" onclick="window.open(this.href);return false; - p72

[CovKid]

No they never are and we'll soon have another 20 pages of oil discussion. 15w40 will be fine.

Just get out there and drive it. Have fun.

[California Dreamin]

Straight forward explanation of the two numbers that make up an engine oil designation:

First number represents the thickness/weight (viscosity) of the oil at 0 degrees F
Second number represents the thickness/weight (viscosity) at 210 degrees F
The designation compares the oils behavior to how different weight 'monogrades' would behave under the same tests.
So: 15W40
At 0 degrees F this oil behaves like a 15 weight 'monograde oil'
And at 210 degrees F the same oil behaves like a 40 weight 'monograde oil' at the same temperature.

Hence why 'modern oils' are classed as 'Multigrades' (one oil that behaves like two viscosity's)

The actual measurement was originally designed by the Society of Automotive Engineers (SAE for short) the machine they used was called a (Redwood Viscometer) This device used a pre-set sized hole by which the 'rate of flow' of the oil was measured at the two temperatures mentioned above.

W was just a designation that was given to all mutigrades to signify their suitability for Winter use (bearing in mind that up until their introduction users were faced with twice yearly oil changes using basic monograde products).

Makes sense?

Martin