Motor oil must begin to circulate as soon as the engine is started. If oil gets cold
enough and begins to solidify, it fails to flow through the oil screen to the pump at engine start and causes
bearings and other critical parts to fail almost immediately.
Pour point is an indicator of the ability of an oil to flow at cold operating temperatures. It is
the lowest temperature at which the fluid will flow.
Modern refining techniques remove most of the wax from petroleum oil, but some wax-like molecules
remain. These wax-like molecules are soluble at ambient temperatures above freezing, but crystallize into a
honeycomblike structure at lower temperatures and cause oil circulation problems.
Pour point depressants keep wax crystals in the oil microscopically small and prevent them from joining
together to form the honeycomb like structure. They lower the temperature at which oil will pour or flow and
are found in most motor oils designed for cold-weather use. As synthetic motor oils do not contain those wax
crystals, they do not require pour point depressant additives.
Since one of an oil’s main functions is to prevent friction and wear,
anti-wear additives are part of the chemical composition of an oil. These additives protect engines by
bonding to metal surfaces and forming a protective film layer between moving parts that are vulnerable to
friction and wear when an engine is first started and before the oil begins to circulate completely. While
this protective film doesn’t entirely eliminate metal-to-metal contact of moving parts at start-up, it
minimizes the effects of contact.
Because excessive engine heat causes chemical breakdown of oil, which in turn results
in permanent thickening of the oil, oxidation inhibitors work to limit the impact of oxidation. Oil oxidation produces acidic gases and sludge
in the crankcase. These gases combine with water in the crankcase to corrode and rust the engine. Corrosion prevention is especially critical in diesel
An oil’s ability to neutralize acids is expressed by its Total Base Number (TBN). The greater the number, the greater the amount of acidic by-products the oil can
neutralize. A high TBN is particularly important in extended drain interval oils, such as AMSOIL motor oils,
because they neutralize acids, and more of them, for a longer period of time.
Most oils for diesel engines in North America have a TBN between 8 and 12. AMSOIL manufactures
several diesel oils with a TBN of 12.
In the same way that some chemical compounds are used to prevent engine rust and
corrosion, other chemicals are added to motor oil to help prevent combustion by-products from forming harmful
sludge or varnish deposits. Detergents are added to motor oil because combustion causes carbon build-up and deposit formation
on the pistons, rings, valves and cylinder walls. Carbon and deposits affect engine temperature, oil
circulation, engine performance and fuel efficiency. Detergent additives clean these by-products from the
oil. Some combustion by-products slip past the piston rings and end up in the motor oil, which can clog the
engine’s oil channels.
While detergents help minimize the amount of combustion by-products,
dispersant additives keep those byproducts suspended in a form so fine they minimize deposits.
They keep the oil in the engine clean while they prevent the build-up of carbon or deposits from burned and
unburned fuel and even from the oil itself. Eventually, these suspended particles are removed by the oil
The addition of silicone or other compounds in very small amounts makes most oils adequately foam-resistant.
It’s important to minimize foaming in motor oil because tiny air bubbles are whipped into motor oil by the action of many rapidly moving parts, resulting in a mass
of oily froth that has very little ability to lubricate or aid in the cooling of the engine. These compounds
weaken the air bubbles, causing them to collapse almost immediately upon forming, allowing the oil to
continue to protect the engine.
All motor oils must be compatible with the various seal materials used in engines. Oil
must not cause seals to shrink, crack, degrade or dissolve. Ideally, oils should cause seals to expand or
“swell” slightly to ensure continued proper sealing.