If it a multi viscosoity oil and rated as a zero weight when it's cold, that's the base oil weight to my knowledge. Just like 10 weight is a base weight, 15 is a base weight and 20 is a base weight.
I think I see where you're going with this, how can a zero weight end up protecting like a 20 weight. You have to go elsewhere, that's above my pay grade.
Here is a pic which is the best I can do but probably doesn't answer the question.
10w means 10winter....as in flows like a 10weight in cold weather startup
Negative... as a dealer you should know that 10w is not a flow test to
simulate start up because oil don't flow much at minus degrees rather the
W is a API test where a cup of oil is stabilized to minus degrees and then
stirred by a magnet and the amount of milli amps it takes to move the
oil are recorded and compared to the chart below...
Quote Amsoil rep Dan Watson who knows what W measures in a 10W test...
"To test how oil will function in cold weather they don't flow the oil
because it would not flow through the orifice instead API takes a cup
and they fill it full of oil and lower it to the temperature required
they stabilize the temp to 0 degrees they set the cup on top of a
magnetic stirrer with a precision motor that turns the magnet. In the
oil is another magnet that magnet suppose to follow the outside magnet
the motor is connected to a precision milli amp meter which measures
amps in milli amps or 1,000 of an amp. the amount of current it takes
to turn the magnet in the cold oil is what is tested they set a range
on amperage if it will turn the magnet and draw 322 milli amps then
that qualifies as a 10w. to test for 5W they go down another 5
degrees and it stills has to achieves that amperage or lower. So what
are they really testing? the ability of the oil to lubricate a moving
part when its very cold will the oil actually allow the part to move
and not channel it can actually move in this cold oil and actually stay
a liquid if it actually does that they want to know hoe well it does
that the little precision motor will tell you how much energy it takes
to make the magnets turn that is a test of the resistance of moving
parts within the volume of oil. that is the trouble of cold oil can
you pump it? can you move it through gears can we get the oil to
"Does that mean its like a 10 weight oil? no they are so different
what API is measuring one measures real viscosity the other measures
apparent viscosity in a laboratory test."
"What the W test tells us is how well this oil function in winter of
cold weather the lower the W number the better it lubricates a 0W
will pump at the lowest temperature for oils right now whereas the
20w doesn't move that well."
"30 is your operational viscosity the 10w is how well it will do below 0"