[email protected]
rec.autos.tech  18 Apr 1992
- > Has anyone ever heard of monitoring the viscosity (or other vital
- Viscosity is one of the more obvious characteristics of motor oil, but
  not directly related to the oil's lubricating ability.  Long chains of
  molecules slide over each other, making oil... oily.  After extended
  use, these chains are broken by heat or simple mechanical shear, and
  viscosity will drop.
- Long before this, the oil's additive package will be full of water,
  gasoline, and combustion byproduct crud.
- Back in ancient times when motor oils were plain old petroleum
  distillates, viscosity was very important.  Any more, it's barely
  relevant.

[email protected]
rec.autos.tech  30 Jul 1992
- >      What are the ramifications of using thicker oil than
  >      that reccomended by manufacturer, especially for
  >      turbo engines. ex: use 20w50 in place of 10w30.
- Unless you've changed the bearing clearances during a rebuild or
  experiencing low oil pressure due to wear, there's seldom any reason
  to change from the type of oil the manufacturer recommends.
- A 20w50 oil does not lubricate any better than a 10w30; it's basically
  just thicker and harder to pump.  In some cases, heavier oils can cause
  low oil pressure at the farther reaches of your oil galleries.

dave williams
rec.autos.tech  05-26-93
- ->      In the past, I've heard recommendations against using
  ->      multi-viscosity motor oils with a "large" viscosity
  ->      range. 20 was usually considered "ok", while 30 was
  ->      pushing it (ie. 10W-30 was better than 10W-40). The
- That's still true to some degree, particularly for cars that are used
  hard.  So-called "turbo grade" oils usually come from a better base
  stock and have fewer additives.  Same for some of the racing oils.
  Unless you're running the car *very* hard, I wouldn't worry about it,
  though.  Modern SG motor oils are pretty tough.
- ->      Now, however, I'm noticing all these new synthetics having
  ->      ranges of 5W-50. Are the rules different for synthetics? By
- Yes, the rules are different for synthetics.  They still use some
  additives for various things, but typically far fewer than conventional
  petroleum based oils.  Some of the early automotive synthetics had
  problems in high-stress applications, but I think they pretty well have
  all that licked now.
- Remember, viscosity is [basically] the oil's resistance to flow.  This
  resistance varies with temperature.  A 10w40 oil flows like a 10wt oil
  when cold, and like a 40wt when hot.  This allows easy starting in cold
  weather, easy pumping in the engine, and takes less horsepower for the
  oil pump.  The oil isn't actually getting thicker as it heats up - it's
  just not thinning out much more than it already is, compared to the
  temp/viscosity curve of an old-style straight stock oil.
- The viscosity has very little to do with how well the oil lubricates,
  at least nowadays.  Back in the [very] old days, the refiners cracked
  crude by very simple processes, and the oil you bought at the service
  station - sometimes in glass bottles - wasn't a hell of a lot different
  than what was pumped out of the ground in the first place.  It broke
  down pretty fast, and oil that went in like molasses when new would
  drain out like water 1500 miles later after all the long-chain molecules
  got ripped apart, etc.  So the viscosity of oil was a way to measure how
  "thick" the oil was, which was fairly proportional to how long it
  would last.  Nowadays, with modern refining and additives, motor oil
  doesn't change much in viscosity as it gets old.