Viscosity

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Wed 23 Nov 2005 17:02
Viscosity - Spospe
Sorry if this is an old one, but I am becoming increasingly puzzled by seeing engine oil for sale bearing the legend ?SAE 0-40? (and sometimes even wider ranges starting from ?0?)

I understand that viscosity is measured in seconds of time for a specified amount to flow through a specified diameter hole, so it is easy to understand ?SAE30?. Multigrade oils quote their viscosity at two temperatures, first the low, then the high. In this case ?SAE (Society of Automobile Engineers) 10-40? means that the oil flows like an SAE10 at low temperature and an SAE 40 at high temperature. But how can oil have a low temperature rating of ?0?

There must be an explanation, but no one that I have asked knows what it is. Do any of the learned readers of The Back Room know the answer?

Thanks ??. Michael
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Wed 23 Nov 2005 17:11
Viscosity - Armitage Shanks {p}
www.honestjohn.co.uk/forum/post/index.htm?f=4&t=33...5 has all the answers and from an expert and two inches above where your post appeared - just joking"
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Wed 23 Nov 2005 17:21
Viscosity - Spospe
www.honestjohn.co.uk/forum/post/index.htm?f=4&t=33...5 has all the answers and from an expert and two
inches above where your post appeared - just joking"<<



I have read the above article mentioned by Armitage Shanks and whilst it is informative, it does not answer my question, which is "how can you have a viscosity of 0?"
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Wed 23 Nov 2005 19:02
Viscosity - martint123
www.carbibles.com/viscosity.html

What about 0W oils then?

Good question. Given that you can't have 50ml, 60ml or 200ml of oil flowing through any size hole in zero seconds, what on earth does the 0W rating denote? Well it's a special case denoting a difference in the 'pour point' of the oil. Most 5W oils have a pour point at -40°F (-40°C) The base oil is the same in 0W40, but it's pour point has been lowered even further - sometimes to as much as -50°F (-46°C)

Pour point is 5°F above the point at which a chilled oil shows no movement at the surface for 5 seconds when inclined. This measurement is especially important for oils used in the winter. A lot of manufacturers tend to quote pumping temperature rather than pour point. Pumping temperature is the temperature at which the oil will pump around the engine and maintain adequate oil pressure. This is typically 20°F above the pour point - ie. 25°F above the point at which the oil is basically a gel.

So 0W oils don't flow through a viscometer in zero seconds - they rate at 5 seconds like a 5W oil, but they will be pourable at a much lower temperature. The bottom line then is that if you think your car is ever likely to see a cold morning in the -45°F (-43°C) range, you should be considering 0W40 oil. If not, 5W40 will do. Note that at -45°F, you'll probably have more to worry about than your engine oil - like your radiator fluid, brittle tyres, frozen locks, permafrost on the windscreen etc.etc.etc.......
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Wed 23 Nov 2005 20:40
Viscosity - Ruperts Trooper
Viscosity is just a rating number - it's not a direct measure as Spospe suggests.

I dare say it won't be long before a -5 rated oil is available which will be even thinner than 0, or perhaps it already is!
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Wed 23 Nov 2005 22:54
Viscosity - mgbv8
Specified per this chart

www.infineum.com/information/viscosity.html
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