From: Mil-Veh Co. (milveh@sbcglobal.net)
Date: Fri Dec 17 2004 - 09:47:20 PST
I know we've discussed this at great length before,
however it is probably worth repeating for those new
to the list or in case you missed before.
The spread of multi-grade particularly in a diesel
engine should generally NOT be far apart as in a 15-40
grade Rotella or otherwise and I know a lot of you
love your 15-40 oil, but please read on...
Binders or polymers are used in the formulation of
multigrades and they don't lubricate, its not their
job. "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.
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".
Are you a believer yet? The bottom line here is the
more binders the less protection you have with a
sheering factor* or at the point where oil can
effectively seperate the two bearing surfaces that it
was designed to lubricate by seperation.
If you still need more info. read on....not all
diesels are created equal and the army has gone to
great length to determine which oils will offer
maximum service. I suggest you always use what is
recommended, forget testimonial evidence, thats junk
is reserved for diet ad claims, not your precious
motor oil.
*At 100C the range of kinematic viscosity falls into
each SAE viscosity grade. This system was developed
years ago and is technically obsolete, however, the
general public is so familiar with it that it remains
today.
How the engine reacts to oil at a high temperature is
measured by the "high temperature, high shear" (HTHS)
viscosity. This value directly correlates to the oil
film thickness in a bearing as I noted above. And this
brings up why you want to use the right oil for your
mil-veh.
If you tend to drive faster or under heavy load, you
will increase the under-hood temperatures and in the
newer military engines there is tighter engineering
tolerances and this makes oil critical. Using the
wrong oil means you have a greater chance at
overworking engine oils.
That overworked oil can change in chemical makeup into
a sort of tar-like substance. This blocks oil channels
in the engine and will likely cause the engine to
seize up, not fun.
So when farmer Ed says, "Heck, I been usin 15-40 in my
diesels for 30 years without a problem when the mfg
called for 30 wt, then its only because those oils
never reached critical load. The difference between
overworked and stable is slight at high operating
speeds and loads, so why risk it just to be different?
Use what the manual tells you too! If your engine
calls for 15-40, fine then use it because the mfg. has
tested your sloppy engine tolerances and puny load
ranges and your underpower-dog of an engine can handle
it,(I'm just kidding with ya) but if the mfg. says use
30 wt. or even a 15-30, then don't be cute and use
10-40!!!!!!! You're only asking for trouble.
If you have finished reading this in its entirety you
now know more about oils than 99% of the public,
congratulations!
--- Bjorn Brandstedt <super_deuce@hotmail.com> wrote:
> 1) Started in less than 15 seconds. Fast revving
> thanks to batteries in top
> condition (coutesy of Chargetek 500). No starting
> aids. Rotella 15-40 oil.
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