In article <rvz2b.90049$Ij4.23983@news2.central.cox.net>,
"Malcom Gillette" <1mgillette1@cox.net> wrote:

> The first # refers to the base weight of the oil.


Oh my.

A multigrade oil has two numbers on it--say 10W30. They mean different
things.

The numbers refer to the oil's viscosity, as we all know. "Weight" in
this case is slang; if you were to put a quart of 10W30 on a scale next
to a quart of 20W50, there might be a few grams difference between the
two but nothing to write home about. They measure viscosity with a
Saybolt Viscosimeter, which is a bucket with a hole in the bottom of it.
Plug up the hole, put in some oil, open the hole and time the oil as it
drains. The longer it takes, the higher the viscosity. Compare the time
to a table that comes with the viscosimeter and you'll learn the oil's
weight.

The "10W" part is the oil's winter (sometimes called "arctic")
viscosity. It's measured at 0 degrees celsius. The viscosimeter is
different, with a larger hole than the summer viscosimeter, and the
table is different--the times are much longer because you're dealing
with a fluid the consistency of honey.

The "30" part is the oil's summer viscosity. It's measured at 100
degrees celsius through a viscosimeter with a little hole in it.

(Having said that, all of which is true, the international standards
community has gotten involved. Now it's all in metric, and certified by
ISO, but the US table's been jiggered around so that 30-weight oil is
still 30-weight oil. Europeans always liked their oil spec'd in Saybolt
times; I have a Heidelberg press that lists the oil it needs in Saybolt
times. Then you call the oil company and ask for some oil with that
special rating, and what shows up? Straight 30-weight Shell Rotella
T...with an invoice for $425 for a 55-gallon drum. We buy it at Sam's
for $257 a drum now.)

You can go to the store, buy a quart of straight-weight oil and run it
through the two viscosimeters so you'll have a "multigrade" oil, but it
might be 750W30. To solve the problem, everyone's on the right track:
take an arctic oil with one viscosity and add polymers to modify the
weight/temp curve.

I remember in my misspent youth running into 20W20 Pennzoil. My dad
loved 20W20 Pennzoil in his Chrysler for some reason. Now he's into 5W30
Mobil 1.

Oh, the 0W oil? The arctic table does go that low. Why you'd want oil
that thin I have no idea.

--jmowreader
a fount of useless knowledge