Oil Specification Theory

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cunning plan

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As oil is specified / categorised with grades, including 'Winter' grades (the small 'w') and these oil grades are measured in viscosity with 0 being the 'thinnest' and the higher the number, the thicker it is, why does warming your engine make an oil change easier because the oil is hot and thinner?

To clarify what I mean, let us look at a 5w 30 specified oil:

That specification shows us that the oil will act like a 5 grade in winter (w), but a 30 grade when the oil has reached temperature (100 degrees C I believe).

On that basis, the oil is thinner cold than it is hot (as 5 is lower than 30)... However we know it is the other way around, oil is thinner when hot, so it should be 30w 5, but it isn't...

There is some lunchtime thinking for you... :rofl:

:popcorn:
 
i dont think its behaviour changes, rather that it has properties that allow it to perform over a range of temps. 'old' style oil had more limited properties and was less of a blend. Hot oil becomes runnier, thats for sure
 
Multigrade oils were mainly produced to reduce the practice of using a thinner oil in winter and a thicker oil in summer, and are simply explained by the fact that the oil falls into 2 viscosity grades, i.e. 20W50 is graded at 20W grade and 50 grade for example.
These were first developed 50ish years ago by the inclusion of polymer components that slow the rate of thinning as the oil warms up (e.g. 20W) and slow the rate of thickening as the oil cools down (e.g. 50)

For an oil to attain the lower specification target, the oil must have a certain maximum viscosity at low temperature. The actual viscosity and the temperature vary with the viscosity grade, but in all cases the lower the number, the thinner the oil.
So a 5W grade oil is thinner than a 10W grade oil at temperatures encountered in UK winter conditions.
This is important because a thinner oil will circulate faster on cold start, affording better engine protection.

For an oil to attain the higher specification target, it must fall within certain limits at 100 deg Celcius. In this case the temperature target does not vary with the viscosity grade, and if there is no "W" stated, the measuring temperature is always 100 deg Celcius.
Again the lower the number, the thinner the oil, so a 30 grade oil is thinner than a 40 grade oil at 100 deg Celcius. 100 deg Celcius is used as it typical of maximum bulk oil temperatures in an operating engine.
High performance engines used in hotter climates are recommended to use heavier grade oils to prevent premature engine wear.

Although rather confusingly, for example, a 10W40 multigrade is permitted to have less viscosity at 100 deg Celcius than a 15W40 multigrade.
 
cunning plan said:
As oil is specified / categorised with grades, including 'Winter' grades (the small 'w') and these oil grades are measured in viscosity with 0 being the 'thinnest' and the higher the number, the thicker it is, why does warming your engine make an oil change easier because the oil is hot and thinner?

To clarify what I mean, let us look at a 5w 30 specified oil:

That specification shows us that the oil will act like a 5 grade in winter (w), but a 30 grade when the oil has reached temperature (100 degrees C I believe).

On that basis, the oil is thinner cold than it is hot (as 5 is lower than 30)... However we know it is the other way around, oil is thinner when hot, so it should be 30w 5, but it isn't...

There is some lunchtime thinking for you... :rofl:

:popcorn:
Bluesnailman said:
i dont think its behaviour changes, rather that it has properties that allow it to perform over a range of temps. 'old' style oil had more limited properties and was less of a blend. Hot oil becomes runnier, thats for sure


Hmm, I am still not sure.

Looking at a 5w 30 Oil again:
If you warm your engine up to 100 degrees C, then perform an oil change, the oil at that point is 'acting' like a 30 grade, but running smoothly.

However, if you did an oil change when the oil is cold, it is acting like a 5 grade, but thicker.

It just does not make sense! :lol:

[youtube]aMr7XxitOkg[/youtube]
 
I'm a bit confused by this thread.

At 100 degrees C, a 0 weight oil would still be thinner than a 30 weight.

The difference between the temperatures (say 10 and 100 degrees C) is the reason the oil is more runny.

Just because 0W30 acts as a 30 weight oil at 100 degrees C, doesn't mean that it'll be less runny than a 0 weight oil at any other temperature.
 
Just put 20w50 in the bloody thing and drive it. Check it often and change it at regular intervals and leave all the science to the man at Shell with a white coat ;)
 
Tofufi said:
I'm a bit confused by this thread.

At 100 degrees C, a 0 weight oil would still be thinner than a 30 weight.

The difference between the temperatures (say 10 and 100 degrees C) is the reason the oil is more runny.

Just because 0W30 acts as a 30 weight oil at 100 degrees C, doesn't mean that it'll be less runny than a 0 weight oil at any other temperature.

I guess I am not explaining the question well enough as I am wondering about the grading system more than the actual oil.

Put simply:

0w grade oil is supposed to be the thinnest / least viscous, so it should run out faster when you drain the oil.

However:

We know that you should warm the engine as the oil will flow faster as it is thinner, BUT at that point, the oil is acting like a 30 grade.

30 grade is thicker than 0W grade, yet the 30 grade comes out faster than the 0W, which is specified to be thinner... :lol:

K@rlos said:
Just put 20w50 in the bloody thing and drive it. Check it often and change it at regular intervals and leave all the science to the man at Shell with a white coat ;)

:lol:

It was more of a 'having a beer looking at the label on an old oil container and wondering' kind of question rather than an actual concern, just thought it might lead to some interesting conversation. :party0047:

:popcorn:
 

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