Re: [alfa] Mixing Oil Viscosities

[Robert Wilkinson <wilk@xxxxxxxxxxxxxxxxx>]

Paul Bayly wrote:

...With modern multi viscosity lubricants available today all you need do
is pick one to suit your requirements.  These oils aren't simply a blend of
different weight oils, the entire volume of oil changes viscosity with
temperature.  So a 10W-50 is ALL 10 cold, and ALL 50 hot (near enough
anyway)....

A "single-weight" oil changes its viscosity with temperature too, in fact
FAR MORE than a multi-weight.  Thus, for example, it's 30 weight at zero
degrees F (and far too thick) and 30 weight at 275 deg F (and far too
thin).  Water, too, about doubles it's viscosity between 212 deg F and 32
deg F (the difference is obvious when you pour it).  Multi-weight oils
change viscosity LESS, although they still thin as they get hotter.  In
other words (considering 10W/50), 50 weight oil when hot is still thinner
than 10 weight when cold, but obviously not as thin as hot 10 weight.  The
nomemclature is somewhat backwards, in that a bigger spread, like 5W/50
compared to 20W/40, actually means a smaller change in viscosity with
temperature. What you want is zero temperature coefficient, which is
evidently impossible with lubricating oils, but polydisperse blends
("multi-weight"; trick molecules (synthetics) or mixtures of different
molecular weights and side chains)are designed to approach this as much as
possible.  Probably so as to make sense long ago when multigrades first hit
the market, manufacturers state what single weight oil their modern oils
are equivalent to at extreme temperatures, rather than the more
straightforward way, which would be to state how insensitive their oil's
viscosity is to temperature.  Another point: the polydispersity breaks down
with time, thereby increasing the temperature sensitivity of viscosity.
Synthetics do this far less than conventional oils.  Sorry to most
(hopefully not all) of you who already know this.

Bob Wilkinson
72 Spider
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