The kodachrome fades quickly now but I recall a good summary of multigrade
oils to mention that the viscosity of the oil is dependent upon the length
of the polymer chains.   Adding "spring-like" polymers to a base oil
provides the multi-grade effect.  Most "big-rig" diesel herders use a single
grade oil often just plain old 40W for most service.  "DELO" was a moniker
that came to mind for this service.

The oil is primarily valuable as a lubricant and as a heat transport
mechanism a secondary effect is as a suspending medium for particulates.
Its viscosity degrades when the polymers get shorter.  This is generally
caused by breaking the polymer mechanically.  Divine guidance, facing mecca,
or use of qualified exorcists will not reduce this degradation.  The
polymers must be meticulously re-joined using your choice of adhesives such
as gorilla snot or Barge's cement OR simply replaced by new longchain
polymers in the form of new oil.

The filter is a dirt catcher.  The more dirt it catches the less dirt or
particulates remains in the oil.  It has VDL to do with the viscosity of the
oil, it has a lot to do with the abrasive qualities of the oil/particulate
mix.  Whether or not one uses Dino or Sintho oil matters little to the
accumulated particles.  Those particles produced as combustion by products
will be there slowly wearing away at the bearing surfaces until removed from
the oil.  The filter helps, changing the oil helps but in the end entropy
wins and the motor wears out.  Just as perfect health is the slowest
possible rate at which one can die, engine health is the acceptable rate at
which a motor can wear out.

So if the filter was perfect and no particles were present, ever, then the
problem would reduce to  polymer destruction and subsequent reduction in
viscosity.  As long as there is an acceptable film of oil on plain bearings
wear is almost non-existent.  Scuffing occurs when the film is broken either
by pressure or by ... particles.  The film degrades as the polymers are
broken.  QED.

At the film lubricated piston ring/cylinder wall the abrasive content of the
air has FAR more effect on wear than the gradual polymer degradation.

SAE has bags and bags of data in the literature on these phenomena.

The vanagon motor is pretty dumb.  It has no knowledge of what kind of
vehicle it is in.  RVC