In reply to a message from Roger’95 sent Tue 14 Jul 2009:
I recall a test in one of the motorcycle magazines (Motorcycle
Consumer News I believe) from several years ago where they tested
10W40 against 20W50. What stuck in my mind was that the 20W50
ended up running hotter and had lower viscosity than did the
10W40. They attributed the temperature difference to the greater
energy requirement to shear the 20W50 molecules between the moving
and static parts. Their test was probably performed on an air
cooled motorcycle engine, which may have had higher overall oil
temperatures than do our liquid cooled Jags.
The recent requirements to improve fuel economy of new cars is
behind the newer recommendations for thinner oils. The improvement
in lubrication technology and metallurgy also makes it possible to
safely use thinner oils.
I use 10W40 in my 5.3 in the upper midwest without any ill effects
thus far. I have not been able to easily find 20W50 in the brand I
use. I do change my oil frequently as other tests from the same
magazine show that viscosity degrades with usage, again due to the
shearing effect. The one oil that showed excellent viscosity
retention was Mobil 1 synthetic (NAYYY). I do not use Mobil 1
because of the cost and my frequent changes.
I would like to see more oil pressure on my gauge to feel better
about it. However, then I remember the racing engine rule of thumb
of 10 PSI oil pressure per 1000 RPM and my engine runs in that
range. Any lower than that is risky to the engine. Any higher
than that just wastes energy driving the oil pump.
As mentioned by others on the list, the Jaguar V12 engines appear
to be quite robust.
Kern–
91 Classic Collection Convt. (NCHNTRS)
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