[Mgs] Syntec 20W 50

[Max Heim]

Yes, I've heard this "tight tolerances" story before, but I wonder how much
of it is true? The same laws of thermodynamics apply to engines old and
new... iron, steel and aluminum behave the same... wear affects both
similarly.

Main bearing clearances, for example -- are they that much smaller on a new
economy car, compared to a 60s or 70s car? I actually doubt it. The
molecular properties of oil have not changed.

The things that really have improved are ring sealing and cylinder wall oil
control, both for emissions reasons. But this is unrelated to oil pressure
or viscosity. Most changes in engine design have been driven by either
emissions or friction reduction -- neither of these logically lead to
particularly tight bearing tolerances.

If you really had to adjust your oil viscosity proportional to bearing
clearances, new engines would use one weight, and you'd have to
progressively change to heavier oils as you accumulated mileage. Does anyone
actually do that (other than the extreme case of a high-mile engine with
chronic low oil pressure)?

I won't dispute your final sentence, as a general statement. But I suspect
there is a tendency for drivers in temperate climates to habitually use a
heavier grade than necessary. Just heavy enough is good; heavier than that
is a power and mileage robber.

I should acknowledge one more argument: that 20W50 is what the factory
specified at the time. My answer to that is that this was the highest
quality oil that was then available. Since then the specification has been
upgraded half-a-dozen times, albeit with some unfortunate repercussions
(ZDDP reduction, specifically).

--

Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the primer red one with chrome wires

on 6/22/11 2:27 PM, Steven Trovato at strovato at optonline.net wrote:

> Max,
> 
> The way it was explained to me is modern cars are built to very tight
> tolerances.  A very thin oil pressurizes the oil system just fine,
> and a metered amount flows though into each place oil is supposed to
> be.  And the theory is, an old car, built with looser tolerances, and
> perhaps showing some wear, needs a thicker oil to do the same
> thing.  It's like pumping water into a vertical perforated pipe
> (sealed, except for the perforations).  With big perforations, all
> the water will come out of the bottom few holes.  Small perforations
> will allow water to flow out of all of the holes.  Replace the water
> with molasses and the large holes will all see some flow.  Try that
> with the small holes and very little molasses will come out, though
> the pressure inside will be high. Therefore, different engines need
> different viscosity oils.
> 
> -Steve Trovato
> strovato at optonline.net
> 
> At 01:36 PM 6/22/2011, Max Heim wrote:
>> There is also a lot of misunderstanding regarding how viscosity relates to
>> oil pressure, and what that actually means. People assume they need a
>> "heavy" oil to maintain oil pressure. A higher viscosity oil is harder to
>> pump and may therefore give a higher pressure reading on the gauge, but it
>> also restricts the rate of oil flow, which may lead to oil starvation at the
>> bearings. What the bearings require is a continuous oil film -- this is a
>> function of flow rate, not pressure. The reason we talk about oil pressure
>> is because this is a property that is easily measured with a gauge -- it
>> isn't a good in and of itself. The measurement of oil pressure is merely a
>> crude substitute for the oil flow rate measured at the bearings, which is
>> unobtainable, from a practical standpoint.
>> 
>> --
>> 
>> Max Heim
>> '66 MGB GHN3L76149
>> If you're near Mountain View, CA,
>> it's the primer red one with chrome wires