The question is whether the oil film is resistant enough to
prevent metal to metal contact when at operating temperatures.
The old school of thought was the greater the shear resistance
the film the better, up to the point where the viscosity of the oil
a hindrance to operation. You used a higher viscosity oil to offset the
inevitable thinning as the oil warms, and multi viscosity oils were
to prevent the oil from turning into pudding at low temperatures.
The current thinking is that while this is important, little
once the oil is circulating at temperature, so increasing viscosity is
value and merely increases drag and lowers fuel economy and power.
the thinking is that the lion's share of the wear occurs at startup,
the oil has drained out of the bearings. For the first few seconds of
the engine operates without lubrication while the pump pressurizes the
Because of this shift in thinking, a modern engine uses
anti-drain back valves
to minimize drainage, and uses *LOW* viscosity oils to allow rapid
at startup. This minimizes the amount of time the engine runs without
This also improves mileage due to reduced drag. Engines now use 5W-30
get the oil to the moving parts as quickly as possible. In addition,
with a wide viscosity rating are to be avoided as the viscositizers have
lubricity and film strength. The *MAXIMUM* recommended range is now 25,
20W-50 or 10W-40 with a range of 30 is to be avoided. Instead, 10W-30
used because of it's superior lubricating qualities, more rapid
startup, and lowered drag.
For a given set of tolerances, a low viscosity oil will yield a
pressure. Given the current understanding of wear, lower pressure within
is irrelevant. Startup lubrication and film shear strength are the items
In addition, very high pressure costs power to operate the pump.
10PSI/1000RPM is overkill. So says my hero, David Vizard.
Rest assured that the debate will rage on, and this thread will
little to settle the matter.
From: Mitchel Seff [mailto:firstname.lastname@example.org]
Sent: May 03, 2005 8:24 PM
To: Robert M. Lang
Cc: Navarrette, Vance; email@example.com
Subject: Re: Re Low oil pressure
I'm using 10-40w Castrol Syntec (yes, I know it's not a full synthetic).
Why should I expect accelerated internal wear because of a different
viscosity rating as long as my oil pressure is normal ?