[VAC] Re: Synthetic oil and oil change intervals

["Carol and Oliver Filippi" <ofilippi@xxxxxxxxxxxxx>]

There is no question that good synthetic oils reduce friction and are more
resistant to breakdown from heat than petroleum based oils.

In fact, they are such good lubricants that traditional wear in or break in
does not (or hardly) occurs.

Synthetic oils are commonly used in heavy duty piston type air compressor
applications with excellent results, virtually eliminating wear.  (Of course
there is some wear, but the life of the compressor is extended
significantly.)  There is a lot of experience in this application, and the
compressor manufacturers and dealers recommend using conventional mineral
oil for the initial "break in", since the synthetic is so good that there
would be essentially no break in (or seating of piston rings, etc.) if the
synthetic oil were used at the outset.  Since there is so little wear, and
the synthetic oil is so stable, oil change intervals are extended
dramatically with no negative effect.

Yes, Corvettes come from the factory with Mobil 1.  However, modern
automobile engines are manufactured to very very close tolerances compared
to several years ago (and most likely a lot closer than air compressors.)
That is why modern automobile engines use lower viscosity (thinner) oil than
those of a few years ago.  The reason is to reduce friction and improve fuel
economy, hence the engines are machined to tighter tolerances to function
well with lower viscosity oil.  Obviously, if an engine is built to tight
enough tolerances, there is no need for mating parts to break in.  Us old
folk remember the old break in ritual with a new car.  Now the "break in"
instructions are, "Don't exceed the speed limit for the first thousand or so
miles."

My suggestion would be to use conventional oil for the first 8,000 miles or
so before switching to the synthetic.  I would also change the conventional
oil every 3000 miles or so prior to the switch to synthetic.

Another feature of synthetics is that they maintain their viscosity over a
wide temperature range - far better than petroleum oils, even with viscosity
improvers.  Thus the synthetic is thin enough when the engine is cold to
flow through the oil passages and still maintain its film strength when the
engine gets hot.

The bottom line is that synthetic lubricants are not only better at
lubricating, but also have a far longer life.

HOWEVER, in an internal combustion engine (unlike the air compressor) there
is still the problem of oil contamination from "blow by" where some amount
of the products of combustion get past the piston rings and contaminate the
oil in the crankcase.  Oil additives are utilized to neutralize these
harmful chemicals, but have a finite capacity. (The additives are consumed
by the contaminates).  Clearly, a tight engine will have less blow by,
thereby  extending the life of the additive and lengthening the life of the
oil.

Synthetic oils, therefore,  need to be changed whenever the contamination
from combustion exceeds a safe level.  Clearly, this is a longer interval
(in a tight engine) than the one for a conventional petroleum based oil -
but not indefinitely.  Note that oil contamination is a bigger problem with
diesel engines due to their much higher compression ratios, typically in the
19 or 20 to 1 range.

Water, one of the byproducts of hydrocarbon combustion, can be eliminated as
long as the engine (and oil) get hot enough for a long enough period of time
to boil off.  Lots of short trips aggravates this problem.  Water, combined
with other combustion byproducts (sulfur and nitrogen oxides, for example)
can produce some nasty chemicals that are not conducive to long engine
life - sulfurous and sulfuric acid, nitrous and nitric acid, etc.

I would expect some sort of readily available and inexpensive test for
crankcase oil to show up on the market one of these days.  Until then,
common sense and reasonable judgment will have to determine the useful life
of a synthetic oil change.

I have not seen a current Corvette owner's manual to see what Chevrolet
recommends for the oil change interval with Mobil 1.  This would certainly
be a good starting point.

If anyone knows the current Corvette Mobil 1 recommended oil change
interval, please e-mail me direct.

I would guess that 5,000 to 8,000 miles would be a reasonable synthetic oil
change interval for a modern tight gasoline engine in normal to heavy duty
use.

Any lubrication engineers out there who would like to comment?

Oliver Filippi
ofilippi@earthlink.net