0w vs 5w cold weather.

[Pelican]

Originally Posted By: Shannow
Originally Posted By: Pelican
I can see how some oil may too thick when hot as in ...W30 vs ...W40 or 50; passages, clearances & tolerances may be too small to allow a thick oil through, but on cold starts the thinner the better IMO. Perhaps someone will explain. Thx

With a positive displacement pump doing the oil, it shifts the same volume of oil every revolution...
with oil in it's pumpable range, it takes the same time to fill the galleries, regardless of "W" rating, largely independent of viscosity.
Look at all the curves I posted above, that's the reason that they flatten out.

The above, for a given engine shows full oil pressure at 6 secs for SAE30 at 30F, 6 seconds for 10W30 at 9F, 6 secs for 5W20 at 12 F.
Put all of these oils at 60F, and they'll all take 6 seconds, because that's the time that it takes the pump to fill the engine.
Drop below the readilyy pumpable range, 10F for the 30, -9F for the 10W30, then the pumpability is starting to creep in.
If you are freezing and above, SAE30, or any of the W ratings will get there the same.
0F, vastly different issue, and you need to consider what you need to start.


Thanks Shannow I didn't know this:
with oil in it's pumpable range, it takes the same time to fill the galleries, regardless of "W" rating, largely independent of viscosity.
I though that a 0W would fill the galleries faster than a 5W or 10W. Thanks again.

 

[BrocLuno]

As long as the pump is turning and the oil will fill the pick-up tube, it all goes the same speed

If it's too cold and won't flow into the pick-up tube, all bets are off.

Safety temp for SAE 30 is 32*F. In the old days, we said it was the "Oil Weight" on the thermometer. 30*F for SAE 30, 20*F for SAE 20, etc. Seemed always safe and always to work... Tractors are hard to start start on 6V in the cold. You have to pay attention to the oil. Thank goodness most of those days and 6V systems are mostly passed

Global warming may make this even less of an issue ...

 

[johnchui1984]

Hi. Does it mean that if the oil is able to flow (assume 32F), the rate of pumping the 0W and 5W is the same? So 0W oil is only beneficial if the place is really cold that 5W couldnt flow?

 

[kschachn]

Originally Posted By: johnchui1984
Hi. Does it mean that if the oil is able to flow (assume 32F), the rate of pumping the 0W and 5W is the same? So 0W oil is only beneficial if the place is really cold that 5W couldnt flow?

If it can't flow to the pump pickup tube. That would happen at a temperature well below freezing.

My old BMW used to sit outside in northern Wisconsin where it got below -25F. Even then the difference between a 5W and 0W was not great, I'm sure the car would still start but maybe with a bit more difficulty. 0W ratings aren't a magic bullet.