Below is the most through answer I have yet received on the
Classic Cars-no-syntheitcs-in-classics-article. Below makes sense to
me. In fact, it makes you wonder what Classic Car was thinking.
Rob Rhodes Columbia, S.C.
72 B
---------- Forwarded message ----------
Date: Fri, 3 May 1996 08:49:02 -0500
From: John Lumley <jll4@cornell.edu>
To: aa101479@dasher.csd.sc.edu
I saw the column by Formhalls and thought it was fairly illiterate. I
teach Automotive Engineering at Cornell, and have restored old cars for
fifty years.
The only real difference between synthetic and natural oils is their
stability. The synthetic oils do not break down as readily, and hence do
not form sludges and acidic compounds as fast or as much. They can safely
be left in much longer. They do not require as many additives to obtain
their properties. In natural oils, the additives are very short-lived.
Aside from that, the most important characteristic of an oil is its
viscosity. As somebody else said on the net, 20W-50 is 20W-50 whether it
is synthetic or natural. The characteristics are specified by the SAE, and
the oil company cannot label it that way unless it meets the SAE
specification.
There are other possible differences - oils generally, especially with
additives, are not what is called Newtonian - that is, if you dip in a
finger and withdraw it, they will form a thread (which water will not), and
if you rub a little between your palms it will be more slippery than an
equivalent amount of molasses, because it is pushing your palms apart
slight as it is sheared. STP is a typical additive of this type - most of
the additives used in natural oils are of this type. STP breaks down very
fast; the others a little slower. However, I don't believe synthetic is
significantly more non-Newtonian than a natural oil.
Now, about old cars: for the past 50 years or so (that is, since the second
world war) bearing clearances have been approximately constant at roughly
0.0015". Most of these manufacturers specified SAE30 oil. If you look at
a chart of SAE30 viscosity vs temperature, you will find that it changes
drastically, being very thick at low temperatures, and thin at high
temperatures. A modern multi-grade oil, such as 10W-30 is no thicker than
10W at low temperatures (-20C), and no thinner than 30 at high temperatures
(100C). An oil like 10W-60 (available in Europe) crosses straight 30 at
about 50C. One of these engines will be better protected at high
temperatures with a multi-grade (even 20W-50) than the manufacturer hoped,
at the cost of a few horsepower lost to friction.
Breaking in an engine depends on bad lubrication - rings will not seat
(wear to match the cylinder walls) if they are too well lubricated.
For much older engines - back in the 1930s and 1920s, the clearances were
somewhat larger, and generally a higher SAE grade was recommended by the
manufacturer. A multi-grade that spans the mfg grade is still a good bet.
The best thing about a modern oil is the detergent. You see nonsense about
detergents breaking free chunks of sludge in old engines, and blocking oil
passages. I don't believe that is possible, and I know of no verified
cases. The detergent gradually puts in suspension the accumulated sludge,
eroding the surface. If the engine is old and filthy, you will have to
change the oil and filter frequently as the stuff goes into suspension,
until the engine is clean. Myself, I would prefer to dismantle it and
clean it.
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