Per a series of articles i rememeber from R&T in the early days of turbos,
when lots of people were cooking oil onto the turbo bearings and ruining
them, the additives which allow an oil to span the multigrade range are
called viscosity index improvers,and VI improvers are susceptible to heat
related breakdown. when they break down, they gunk up the rings and stuff
(stop me if the language is too technical) Hence the trade off between
straight weight oils, whcih aren't at all flexible and 0W50 which has so much
VI index improver as to be unreliable under normal conditions. (which is why
you don't see it except perhaps in synthetics) The point of the article in
those early turbo days was that a 10W-40, with more VI improver is also more
susceptible to heat breakdown and so less desireable than a 10W-30. Most new
cars don't recommend anything thicker than 10W30. Because STP is basically
straight VI improver, it is also susceptible to heat breakdown and not
something you want to put in a hot running engine.
having said that, I'm sure the lousiest oil you can buy today is much better
than the best oil available when the A series was designed.
At the risk of changing the direction of the thread, did anyone see Patrick
bedard's article in C&D about oil to use in "collector cars"? The engineer he
spoke to recommended oil rated for use in compression engines for cars that
(like mine unfortunately) are driven infrequently, because those oils have
more anticorrosion additives (to combat the sulfuric acid resulting from
diesel fuel combustion) Most of those oils also have a short multigrade span
(15W-40) because diesel/compression engines are prone to hot spots due to the
high compression ratios.
Dave Groundwater
70 Midget
In a message dated 10/23/99 5:19:06 PM Central Daylight Time,
soavero@yahoo.com writes:
> A thing about multigrade oils is that the
> > two numbers refer to
> > respective viscosity at different temperatures - I
> > think that is the
> > simpliest summary I can manager without running to
> > 2,000 words. Here is the
> > advantage of the 5/50 and 15/50 Mobil 1 synthetic
> > as oppose a 20/50 which is
> > actually thicker at a cold temperature while not
> > getting any thinner at the
> > top end (50). A synthetic also does not break down
> > at very high
> > temperatures, as found in turbocharger centre
> > bearings and extreme operating
> > conditions (maybe low oil pressure and high engine
> > temperature). I don't
> > think STP does any harm to an engine with low oil
> > pressure unless it increase
> > the viscosity at extreme low temperatures (like in
> > the Artic circle?). Some
> > additives, not sure about STP contain additives that
> > do more than just
> > increase viscosity and protect bearings when the oil
> > film either breaks down
> > (ZDDP?) or prevent the break down (Teflon?).
> >
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