Explaination of pumping viscosity -
Once an engine has been started, it needs a continuous and adequate supply of oil. The pumping viscosity of an oil determines how easily an oil will flow from the oil pump to critical parts of the engine in cold temperatures.
There are two types of pumpability problems. If the oil is not sufficiently pumpable (a condition known as being flow-limited) the oil may flow but not rapidly enough to prevent progressive damage to the areas needing lubrication during low temperature starting and operation. If the oil is not pumpable at all at lower temperatures (a condition known as air-binding or gelation) the engine can be severely damaged in a relatively few minutes.
A flow-limited oil may not be adequately pumped because of having too high a viscosity at the low temperature at which the engine is started. To measure the pumping viscosity of an oil, ASTM test method D4684 is used. In this test, a rotor is inserted into a stator containing the oil in question and very slowly cooled at a constant rate over approximately a two-day period until it reaches a specific temperature. A small force is then applied to turn the rotor and the rate at which the rotor turns is related to the viscosity. The unit of measurement used for reporting pumping viscosity is centipoise (cP).
SAE has set specifications for pumping viscosity associated with the 'W' grade of oil as follows:
code:
Viscosity Grade Maximum Pumping Viscosity
25W 60,000 cP at -15°C (+5°F)
20W 60,000 cP at -20°C (-4°F)
15W 60,000 cP at -25°C (-13°F)
10W 60,000 cP at -30°C (-22°F)
5W 60,000 cP at -35°C (-31°F)
0W 60,000 cP at -40°C (-40°F)
Now, is it more important than the other.. let me repost something I wrote earlier which I think will give you a better picture on looking at motor oils.
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To base an oil's performance on base stock's performance is not always the best thing. Kinda like taking an musical band with all 20 or so musicians. singlarly, each one may be the top of the line in playing on time, and with precision. But then put a single drummer in that can't keep a beat, maybe even tends to drown out the instruments, then see how as a whole the band works to gether.
Like the band, the pumpability is one aspect of how well the base stock is going to hold up to cold weather and as to how well the band is going to sound based on one instrument or how well an oil is going to hold up based on one aspect of the base oil is not a true picture.
There is several stages to look at when looking at oil.
1.. Base stock's ability to..
a. hold up to heat (noack)
b. pumpability at cold temps
c. maybe flashpoint
d. operational viscosity
e. pumping viscosity
f. shear stability
2.. detergent packages...
a. how they hold up to acid resisitance
(tbn and detergent levels)
3.. Wear protection...
a. what kind of barrier additive is used
b. what levels of barrier additives(amounts)
These are just some of the things to look at, now, put it all together. If they play off key because one is higher pitched than the other(to much of one additive,not enough of the other), or off beat from the other band members(not playing well with other additives due to clash), Then you have an ok sounding band or maybe even a really sorry sounding band that only 10 beers could make them sound better(use an after market oil additive).
Like many aspects of oil, it along with other parts play a small role but put to gether, like the music band, it all will sound like sweet wonderful music when they all play together (like a properly blended oil) and that my friend is how you look at oils and determine if they are what you want, but listening to the end results being played by the band(doing oil analysis on your engine with your choice of oil) is the key to determining oil quality. This is one reason I provided an area for oil analysis to be shared so that many could see real life #'s instead of just bench #'s.
