Oil Weight

by George Lyle


> Does 2 quarts of 30w + 2 quarts of 50w (same brand) = 4
> quarts of 40w? Oil Weight

Maybe, maybe not, as the viscosity range indicated by the SAE numbers is fairly wide and the numbers themselves are quite arbitrary. Read on if you want to know more.

The way oil viscosity is measured is with a Saybolt orifice viscometer, which is basically a cup surrounded by a water bath which keeps the temperature of the oil sample constant. There is a plugged opening in the bottom of the cup, which is opened when the oil is at the proper temperature. The oil is captured in a 60ml flask and the amount of time required to fill the flask is recorded.

A SAE30 oil takes 58-70 seconds to fill the flask at 210F,
an SAE40 oil takes 70-85 seconds, and
an SAE50 oil takes 85-110 seconds.

There is also a test for low-temperature viscosity that is designated by a "W" suffix on the SAE viscosity rating.
A SAE5W oil takes less than 6000 seconds to fill the flask at 0F.
An SAE10W oil takes 6000-12000 seconds, and
an SAE20W takes 12000-48000 seconds.

I'd assume that the winter oil testing leaves the test engineers with plenty of time to relax while waiting for the test to conclude! Ah, the life of a petroleum engineer!

A "multiviscosity" oil is simply an oil that has been rated using both tests. Nothing magic about it, except that sometimes the composition of the oil has been tweaked to flatten the viscosity-temperature curve.

Note that a SAE30 oil and a SAE30W oil are not the same thing, as they are measured using different tests.


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