How to tell the weight of an oil

[wantablaster]

Yes I know this sounds like a stupid question but I need to know how I figure out the weight of the oil. Does it go by the last number like in 10w 30 oil the oil would be 30 weight or by the difference of the two numbers as in a 10w 40 oil it would be a 30 weight oil. Thanks for any help.

[Rico]

WHAT DO THE NUMBERS MEAN?

Most people believe that a 5w30 oil is good for cold weather use because it is a "5 weight" oil in cold
temperatures and a "30 weight" oil at high temperatures. On the surface this might seem to make a
certain amount of sense. Naturally, a "5 weight" oil would flow better than a "30 weight" oil. This would
make it ideal for cold temperature operation.
Nevertheless, this is a profound misunderstanding of what the labeling means. The two numbers really
have little to do with each other. The final number based upon the kinematic viscosity at 100 degrees C,
MONOGRADE OILS
MULTI-VISCOSITY OILS
as we discussed for monograde oils.
So, if a multi-grade oil, when heated to 100 degrees C, falls within a certain kinematic viscosity it is
classified as a certain SAE grade (the last number - like the "30" in 5w30). In other words, the kinematic
viscosity of a 5w30 multi-viscosity oil falls within the same range at 100 degrees C as a monograde
SAE 30 weight oil does.
A multi-viscosity oil also has to meet a "High Temperature/High Shear" requirement, but I'll talk about
that in a minute.
The first number (the "5" in 5w30) is only a relative number which basically indicates how easily it will
allow an engine to "turn over" at low temperatures. It is NOT a viscosity reference. In other words, a
10w30 is NOT a 10 weight oil in cold temperatures and a 30 weight oil in warm temperatures.
In fact, since SAE viscosity classifications only apply to an oil at 100 degrees C, it doesn't even make
sense to label it as a certain SAE viscosity at any temperature other than 100 degrees C.
Besides, if you thought about it for a second, it wouldn't make sense for a 10w30 oil to be a 10 weight
oil in the cold and a 30 weight oil in warm temperatures. What liquid do you know of that gets "thicker"
as its temperature increases or "thinner" as the temperature decreases?
I would venture to say you probably can't come up with one. This holds true for motor oil as well. If a
10w30 was a 30 weight oil at 100 degrees C and a 10 weight oil at cold temperatures, that would mean
it "thinned out" as the temperature dropped. That just doesn't make any sense considering what we
know about liquids. It just doesn't happen like that.
The fact is that a 5w30 motor oil is thicker in cold temperatures than in warm temperatures. However,
a 5w30 motor oil will be thinner than a 10w30 motor oil when subjected to the same low temperature
conditions - because the "W" number is lower. This is an indication of better cold weather performance.
In other words, a 5w30 flows better in cold weather than a 10w30 motor oil will. Think of the "W" as a
"winter" classification instead of a "weight" classification.
Results from the Cold Crank Simulator (CCS) and Mini-Rotary Viscometer (MRV) tests are used to
determine the oil's "W" grade. The better the engine "startability" of the oil at low temperature, the lower
the W classification. Each W grade must meet certain "startability" requirements at a specified
temperature.
For instance, a 0W grade oil must have a maximum CCS centipoise (cP) value of 3250 @ -30 degrees
C as well as a maximum MRV cP of 60,000 @ -40 degrees C. A 5W grade oil must have a maximum
CCS cP value of 3500 @ -25 degree C and a maximum MRV cP of 60,000 @ -30 degrees C. The
lower the cP value for both specifications, the better.
Notice that the 0W grade oil is tested at a lower temperature on both tests AND must still have a lower
CCS cP value than a 5W oil which is tested at a higher temperature. As a result, a 0w30 will allow your
vehicle to start easier on a cold morning than a 5w30 will. Likewise, a 5w30 oil will pump easier in cold
temperatures than a 10w30 oil will.
Nevertheless, at 100 degrees C, they all fall within the same kinematic viscosity range. Therefore, they
are all classified as SAE 30 weight oils at 100 degrees C. In other words, after your engine has warmed
up, a 0w30 and 10w30 motor oil are basically the same thickness (within a certain SAE specified
range).

[Johnny B]

huh! very long post but very interesting too. i kind of understand about the (w) thing but what does the last number mean? i use 10w40 im my quad. why does that keep it cooler than a 10w30?

[sakirocket28]

From what i understand, the last number in the sequence could basically broken down to the thickness of the oil. a 30 weight oil is thicker than a 40 weight oil.

i use 10w 30

[ATC350X 85]

w means winter. the 5 means how it will flow at 5degrees in the winter basically, 5 will flow the easiest. The 30 is the overall weight classification of the oil.

[tfuchs]

The Oil Multi weight viscosity is determined by the oils ability to thin out slower like a higher viscosity oil. A 10 w30 will start up like a 10w and thin out like a 30w oil woild do ay operating temperatures, due to polymers added to the oil to make it a multiweight.

Hope this helps