Read these WAS OIL Re: Synthetic oil for I-5s

[Carol, Dave, and Zach Weiss]

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Oh, lord, I come back to the list and hit another synthetic oil thread.
:)

I used to make the additives that go into motor oil and so have a little
background, tempered by quite a while out of the business.

Two of the issues about synthetic are viscosity breakdown (syn is much
better) and seal swell (used to be a problem w/ older engines).

Viscosity index is the relationship of viscosity to temperature.
Low(er) viscosity is good when you start up, because the oil flows more
quickly to where it needs to go to lubricate the engine.  When oil gets
hot, its viscosity decreases.  Dino oil base stock that flows ok at
start-up tends to get too thin at normal operating temperature, and so
it needs an additive called viscosity index improver that mitigates the
drop in viscosity with increasing temperature.  At least some VII's
break down over time-the shear in an engine shortens the VII molecules
and they do not have as pronounced an effect as when the oil is fresh.
Synthetics have a flatter viscosity vs. temperature curve than
conventional base stocks, and need less (and often no) VII to meet the
multigrade viscosity specs like 10W30.  They also protect the engine
better before it warms up.  Synthetics used to advertise longer drain
intervals (justifying higher price) because they were much more
shear-stable (less or no VII required), and because they used a higher
percentage of other additives so the additives weren't depleted as
quickly.  I've been out of the business too long to know what the
current treat rates are, but both the oil and car manufacturers
recommend not to exceed the car manufacturers' recommended drain
intervals with any oil.  You could test the oil in your engine over
time, but it's easier for most folks just to change it.

The old wives' tale about not changing an old engine to synthetic
probably came about because some old wives got upset at their gear-head
hubbies that changed to syn and found they were using more oil, because
the synthetic didn't make seals swell as much as dino oil did.  If the
seals swell and are worn away by rotating parts, and then the swelling
decreases.  Note that some elastomer elongation is caused by additives,
esp. those containing nitrogen.  Since then oil manufacturers have
changed both the synthetic bases and the additives packages.  I don't
have data on current synthetic (lack of) seal swell, but I hear it's not
such a problem-perhaps conventional oils cause less swelling now, as
well?

Finally, additives packages weren't necessarily chemically
compatible-some of the additives from different manufacturers would
interact and cause solids to form.  We had tests for additive
compatibility; it was a Really Big Deal.  Supposedly this is better now,
but I still stick to a single brand of oil in any given engine.  I use
M1 over other synthetics because it's available all over.

Dave Weiss
V8 5-speed


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