Issues with 10-40

[Russ Vaughan <russ@xxxx>]

Hi group, for those of you who asked, I quickly found
this article in a web search. I took the applicable
excerpt, but those who want to read further, the
entire text is at:

http://www.vtr.org/maintain/oil-overview.html


"Choosing the best motor oil is a topic that comes up
frequently in discussions between motorheads, whether
they are talking about motorcycles or cars. The
following article is intended to help you make a
choice based on more than the advertising hype. 
Oil companies provide data on their oils most often
referred to as "typical inspection data". This is an
average of the actual physical and a few common
chemical properties of their oils. This information is
available to the public through their distributors or
by writing or calling the company directly. I have
compiled a list of the most popular, premium oils so
that a ready comparison can be made. If your favorite
oil is not on the list get the data from the
distributor and use what I have as a data base. 


Viscosity is a measure of the "flowability" of an oil.
More specifically, it is the property of an oil to
develop and maintain a certain amount of shearing
stress dependent on flow, and then to offer continued
resistance to flow. Thicker oils generally have a
higher viscosity, and thinner oils a lower viscosity.
This is the most important property for an engine. An
oil with too low a viscosity can shear and loose film
strength at high temperatures. An oil with too high a
viscosity may not pump to the proper parts at low
temperatures and the film may tear at high rpm. 

The weights given on oils are arbitrary numbers
assigned by the S.A.E. (Society of Automotive
Engineers). These numbers correspond to "real"
viscosity, as measured by several accepted techniques.
These measurements are taken at specific temperatures.
Oils that fall into a certain range are designated 5,
10, 20, 30, 40, 50 by the S.A.E. The W means the oil
meets specifications for viscosity at 0 F and is
therefore suitable for Winter use. 


Multi viscosity oils work like this: Polymers are
added to a light base (5W, 10W, 20W), which prevent
the oil from thinning as much as it warms up. At cold
temperatures the polymers are coiled up and allow the
oil to flow as their low numbers indicate. As the oil
warms up the polymers begin to unwind into long chains
that prevent the oil from thinning as much as it
normally would. The result is that at 100 degrees C
the oil has thinned only as much as the higher
viscosity number indicates. Another way of looking at
multi-vis oils is to think of a 20W-50 as a 20 weight
oil that will not thin more than a 50 weight would
when hot. 

Multi viscosity oils are one of the great improvements
in oils, but they should be chosen wisely. Always use
a multi grade with the narrowest span of viscosity
that is appropriate for the temperatures you are going
to encounter. In the winter base your decision on the
lowest temperature you will encounter, in the summer,
the highest temperature you expect. The polymers can
shear and burn forming deposits that can cause ring
sticking and other problems. 10W-40 and 5W-30 require
a lot of polymers (synthetics excluded) to achieve
that range. This has caused problems in diesel
engines, but fewer polymers are better for all
engines. The wide viscosity range oils, in general,
are more prone to viscosity and thermal breakdown due
to the high polymer content. It is the oil that
lubricates, not the additives. Oils that can do their
job with the fewest additives are the best. 

Very few manufactures recommend 10W-40 any more, and
some threaten to void warranties if it is used. It was
not included in this article for that reason. 20W-50
is the same 30 point spread, but because it starts
with a heavier base it requires less viscosity index
improvers (polymers) to do the job. AMSOIL can
formulate their 10W-30 and 15W-40 with no viscosity
index improvers but uses some in the 10W-40 and 5W-30.
Mobil 1 uses no viscosity improvers in their 5W-30,
and I assume the new 10W-30. Follow your
manufacturer's recommendations as to which weights are
appropriate for your vehicle. 





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