Not so, it is film resistance that protects bearings. Whether an engine
needs a high viscosity or a low viscosity depends on its specified bearing
clearances, higher needing higher and vice-versa. Higher flow might well be
achieved using a low viscosity oil in a high clearance engine, but the film
will break down easier, not to mention give lower oil pressure on a gauge.
Worn high clearance engines give low oil pressures with the correct grade of
oil, so when you see that and add a thickener and are satisfied when you see
a higher pressure on the gauge, you are only kidding yourself and actually
making things worse as you have reduced the flow and increased the
likelihood of metal to metal contact in the bearings. Conversely too high
an oil pressure, either by uprating the pump which is another thing some V8
owners do or using too thick a grade on a modern engine, causes bearing
erosion. An oil pressure gauge is largely something else for owners to fret
about unnecessarily, only if you become aware of a gradual reduction in
pressure over time can it be said to be marginally useful, for a sudden and
total loss of pressure it isn't as useful as a warning light, which is why
cars these days rarely have a gauge.
And heavier than optimal for what? 20W/50 has always been specified for our
engines, along with 10W/50 and 10W/40 *for temperate climates*. For cooler
climates it must be 10W/30, 10W/40 or 10W/50, only for all temperatures
below -10C/15F should it be as low as 5W/20 or 5W30.
PaulH.
----- Original Message -----
> Just to throw another spanner into the works, there is an argument that
> 20w50 is heavier than optimal, and that higher effective oil flow is
> achieved using lighter weight synthetic oils such as 5w20. Oil flow is
> what
> protects bearings -- oil pressure is just a crude indication of oil flow,
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