AOPA Pilot Magazine - October 2020 - 99

OUR ENGINES depend on both hydrodynamic and boundary lubrication.

thicker as temperature decreases, just not
nearly as much. To make a multigrade oil,
you start with a thin monograde oil and then
add an artificial thickening agent called a
viscosity index improver. This man-made
polymer has the unusual property that it gets
thicker and more viscous when heated-the
opposite of what mineral oil does.
Viscosity index (VI) is the rate at which
viscosity changes as temperature increases.
The higher the VI, the less the oil thins as it
heats. A multigrade aircraft oil rated as 15W50, for example, is basically SAE 15 oil to
which roughly 10 percent VI improvers are
added to obtain the viscosity of SAE 50 oil
at 100 degrees Celsius (212 degrees F) while
maintaining the pourability of SAE 15 oil at 0
degrees C (32 degrees F). At operating temperature, Aeroshell W100 and 15W-50 have
essentially the same viscosity. At room temperature or colder, the difference in viscosity

is dramatic. The W100 pours like ketchup,
while the 15W-50 pours like tomato juice.
ADDITIVES

Oil additives are vital for both lubricity
and longevity. Without them, the oil would
become contaminated, break down, leak
out, or not properly protect engine parts.
We've already discussed two kinds of additives found in piston aircraft engine oils: VI
improvers modify base oil viscosity to create
multigrade oil. Anti-wear and extreme-pressure additives chemically react with metal
surfaces to create a protective barrier that
inhibits micro-weld formation and facilitates boundary lubrication.
Another class of additives called friction
modifiers improves lubricity by making the
oil more slippery. Antifoam agents modify
the surface tension of oil to allow oil bubbles
to burst more readily, minimizing foaming.

MINERAL OIL molecules are "branchy," synthetic oil molecules are smooth, and viscosity
index improvers roll up into a ball when cool and unroll at high temperatures to become
more viscous.

Corrosion inhibitors also reduce
rusting of steel engine parts (e.g., crankshafts, camshafts, lifters, cylinder barrels)
by forming a chemical barrier to repel
moisture from metal surfaces. Tackiness
agents increase oil's adhesive properties to
improve film retention and better protect
these parts, while acid neutralizers protect
them from attack by acids that build up in
engine oil when moisture and combustion
byproducts combine. Oxidation inhibitors
protect the oil itself from premature aging
due to oxidation at high temperatures.
Detergents and dispersants help keep
engines clean and free of deposits by
holding combustion contaminants in suspension and preventing them from forming
sludge and varnish deposits inside the
engine. Detergents are typically metallicbased chemicals and are often found in
automotive oils but can cause problems
in aircraft engines. Consequently, all modern piston aircraft engine oils use ashless
dispersants instead of detergents, which is
why they are referred to as AD oils.
OIL RECOMMENDATIONS

All modern piston aircraft engine oils are
excellent, and will do a good job if you change
the oil and filter regularly (at least every 50
hours or four calendar months, whichever
comes first) and keep your oil temperature in
the sweet spot (180 degrees F to 200 degrees
F indicated) as much as possible.
Personally, I prefer using a monograde
oil like Aeroshell W100 unless you operate in cold temperatures where multigrade
is required. I don't like synthetic or semisynthetic oils (e.g., Aeroshell 15W-50) for
engines that run on leaded avgas, although
synthetics are great if you run your
engine on unleaded mogas. Finally, I'm
a fan of the aftermarket oil additive ASL
CamGuard because of its outstanding antirust and anti-wear properties. I've been
using Aeroshell W100 with CamGuard in
the engines of my Cessna Turbo 310 for
decades and the longevity I've obtained
from those engines has been the stuff of
legend.
AOPA
MIKE BUSCH is an A&P/IA.
EMAIL mike.busch@savvyaviation.com

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AOPA Pilot Magazine - October 2020

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