Engine oil grading
Joined: Aug 2000
Posts: 1,966
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From: Norfolk
100 and 80 are quite easy to explain:
At a certain temperature, a given quantity of the oil will flow through a predetermined diameter hole in a metal plate in 100 seconds or 80 secs respectively!
So the 80 is less viscous, ie more runny than the 100 and is often used in winter time.
Apologies for the vagueness but it is a long time since I had to know the exact temp, quantity and hole bore!
The 20-50 is a multigrade but these appeared long after my time!!
At a certain temperature, a given quantity of the oil will flow through a predetermined diameter hole in a metal plate in 100 seconds or 80 secs respectively!
So the 80 is less viscous, ie more runny than the 100 and is often used in winter time.
Apologies for the vagueness but it is a long time since I had to know the exact temp, quantity and hole bore!
The 20-50 is a multigrade but these appeared long after my time!!
Joined: Dec 2004
Posts: 832
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From: East Anglia
To follow on from what Stik said, the viscosity/runnyness changes with temperature so W100 will run through a hole faster at high temp than low temp. There is a known change with temperature.
Multigrade oils are designed to be runnier at low temp than they should be and thicker at high temp. Therefore the use of two numbers 10-40 etc. The 10 refers to the viscosity at low temp and the 40 the viscosity at high temp.
The advantage of a multigrade is that it is thinner when cold and therefore circulates quicker. It is thicker at high temp and therefore works better there than a non multigrade.
5-50 multigrade is 'better' in this regard than 10-40.
I'm sure someone more competent will explain all of this using terms like Stokes-Reynolds numbers etc.
Multigrade oils are designed to be runnier at low temp than they should be and thicker at high temp. Therefore the use of two numbers 10-40 etc. The 10 refers to the viscosity at low temp and the 40 the viscosity at high temp.
The advantage of a multigrade is that it is thinner when cold and therefore circulates quicker. It is thicker at high temp and therefore works better there than a non multigrade.
5-50 multigrade is 'better' in this regard than 10-40.
I'm sure someone more competent will explain all of this using terms like Stokes-Reynolds numbers etc.
Joined: Apr 2003
Posts: 71
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From: UK
Not mentioned so far. W at the front of the number eg. W80 or W100 is for WASHING. 80 or 100 (without W!) is straight oil.
You should discuss with your engineer what oil you are using - straight or washing. You may need to use straight after a rebuild or overhall. If straight has been used for sometime then it may not be a good idea to then switch to using washing.
DISCLAIMER, I am NOT an engineer, just owner. This as I understand it applies to 80 100 etc. Not sure about the multi-grades. You should discuss what you are using with your engineer as swapping, changing, mixing could cost you!!
You should discuss with your engineer what oil you are using - straight or washing. You may need to use straight after a rebuild or overhall. If straight has been used for sometime then it may not be a good idea to then switch to using washing.
DISCLAIMER, I am NOT an engineer, just owner. This as I understand it applies to 80 100 etc. Not sure about the multi-grades. You should discuss what you are using with your engineer as swapping, changing, mixing could cost you!!
Joined: Feb 2001
Aviation Qualifications: PPL
Posts: 1,222
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From: Worcestershire, UK
An oil's viscosity measures its ability to flow. Viscosity decreases as temperature increases. The rate of this change is known as the viscosity index (VI). The higher the VI, the lower the rate of change of viscosity. Polymer-based additives have been developed to improve the VI of lube oils.
An engine lubricating oil needs to have a low enough viscosity to flow readily at low temperatures, and a high enough viscosity to provide suitable lubrication at normal working temperatures. Accordingly, multigrade oils have been developed. These act as a low viscosity oil at cold temperatures, and a high viscosity oil at higher temperatures. Basically, a SAE5W50 oil would act like a SAE5W oil at low temperatures, and like a SAE50 oil at higher temperatures, thus ensuring adequate lubrication throughout the operating range of temperatures.
An engine lubricating oil needs to have a low enough viscosity to flow readily at low temperatures, and a high enough viscosity to provide suitable lubrication at normal working temperatures. Accordingly, multigrade oils have been developed. These act as a low viscosity oil at cold temperatures, and a high viscosity oil at higher temperatures. Basically, a SAE5W50 oil would act like a SAE5W oil at low temperatures, and like a SAE50 oil at higher temperatures, thus ensuring adequate lubrication throughout the operating range of temperatures.
