What's the difference between 15w and 20w oil?

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Oct 22, 2003
By Detroit
The API Engine Oil Classification chart for 2002 show both 15w and 20w classes as ranging upwards of 5.6 viscosity (cSt at 100C). The next range starts at 9.3 for 25w and 30 weight. So it seems that both 15w and 20w range from 5.6 to 9.3. Is there a split in that range below which is 15w and above which is 20w? Or is the difference in HT/HS viscosity, but the chart shows 15w, 20w, and even 25w all having high-sheer-rate viscosity (cP at 150C min) of 3.7. Am I confused or is it really pointless to get a 20w oil because the 15w will be just as good?
I think its pointless to get either one. Why do you want a 15W or 20W (assuming a multi-vis) for poorer flow at startup conditions?
I'm a little confused. A W designates the 2 weights listed as being suitable for winter. Hence they would not be tested at 100C.

To answer your question...a 15W is suitable down to -20C (-4F) and a 20W down to -15C (5F).

And Al, to answer you question...as long as you fall within the required temperature range you will be fine. It's just as pointless to run a 0W anything in Mexico, AZ, Aust. etc. as it would be to run a 20W in Winnipeg, AL, Antarctica, etc.. That's why we have these ratings...to stipulate the "operating range" of a specified oil...either it works or it doesn't ie. it's solid because one has exceeded the usuable low range (cold properties)...
The API chart lists a low-sheer-rate kinematic viscosity at 100C (cSt) for all grades from 0w through 25w and 20 wt through 60 wt. My assumption is that the W part likely is pretty close to the base oil. So the 100C viscosity of the W part should give you an indication on how robust the oil is under high temp. I suspect that the thicker the base oil, the higher the high-sheer-rate viscosity. So I am wondering if there is a difference between 15w and 20w in 100C viscosity of the base oil and/or in HT/HS. Or maybe the difference shows up some other way.
TallPaul, looking at the chart again...I think those are minimum values and cannot be extrapolted across the board to all 15 and 20W's. The viscosity spread ultimately wins (all things being equal).

However, a synthetic 15W possibly even having a higher Low temp. HT/HS than a 20W conventional.

Originally posted by Al:
I think its pointless to get either one. Why do you want a 15W or 20W (assuming a multi-vis) for poorer flow at startup conditions?

Well, in a moderate climate, and if one is more interested in less consumption and seepage with a higher viscosity oil then this is a reason to use a 15W. Further, I don't necessarily buy into the less wear at start up with a 0W etc.

That is why I am switching to a 15W40 in one high mileage engine in lieu of its life long 10W30. I also believe a 15W is more stable then a 10W40, fewer VI etc.
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