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Path: utzoo!linus!decvax!harpo!eagle!alice!rabbit!ark
From: ark@rabbit.UUCP
Newsgroups: net.auto
Subject: Re: Magic Oil Numbers
Message-ID: <1795@rabbit.UUCP>
Date: Sat, 13-Aug-83 12:45:39 EDT
Article-I.D.: rabbit.1795
Posted: Sat Aug 13 12:45:39 1983
Date-Received: Sat, 13-Aug-83 20:14:25 EDT
References: <2849@decwrl.UUCP>
Organization: Bell Labs, Murray Hill
Lines: 31

I'm not completely sure about this, but I believe that oil rating numbers
are a measure of viscosity.  The higher the number, the gooier the oil.
The numbers are measured by timing how long it takes a standard volume
to flow out of a standard sized hole at a standard pressure and temperature.
For instance, the SAE 90 oil used to lubricate the gearboxes of some
marine outboard motors has approximately the consistency of honey.

For use in an automobile engine, there's a tradeoff.  The heavier the oil,
the better a job it will do of protecting your engine from wear, but
the more energy will be lost sloshing the oil around.  Manufacturers
tend to recommend a particular grade as what they believe to be the
best tradeoff for their engines.

Ordinary oil's viscosity changes with temperature -- the colder, the gooier.
I guess the rating numbers take this into account.  Thus, SAE 30 oil will
pour readily in summer but be quite sludgy in winter.  This sludginess
would make a car using it quite hard to start, because the starter has
to slosh all that heavy stuff around to get the engine going.

Thus, many modern oils are formulated (don't ask me how!) so as to have less
of a viscosity change with temperature.  Their rating has two numbers
separated by a W.  The W stands for "winter":  a 10W40 oil normally behaves
as SAE 40, but in winter its viscosity stays as low as a straight SAE 10 would.

These multigrade oils are therefore to be much preferred if you ever have
to start your car in cold weather.

Incidentally, every car I've ever owned or driven extensively has had
10W40 recommended by its manufacturer.  Most "premium" oils such as
Exxon Uniflo, Castrol GTX, Mobil 1 (an incredibly expensive synthetic)
are rated 10W40.