Oils are graded according to their viscosity by a
series of Society of Automotive Engineers (SAE)
The viscosity of the oil increases
progressively with the SAE number. An SAE 4 oil
would be very light (low viscosity) and SAE 90 oil
would be very heavy (high viscosity). The viscosity of
the oil used in internal-combustion engines ranges from
SAE 5 (arctic use) to SAE 60 (desert use). It should be
noted that the SAE number of the oil has nothing to do
with the quality of the oil.
The viscosity number of the oil is determined by
heating the oil to a predetermined temperature and
allowing it to flow through a precisely sized orifice
while measuring the rate of flow. The faster an oil
flows, the lower the viscosity. The testing device is
called a viscosimeter. The viscosity of the oil is printed
on top of the oil can. Oil viscosity is written SAE 10,
SAE 20, SAE 30, and so on. The letter W will follow
Figure 6-19.Sources of oil contamination.
any oil that meets SAE low-temperature requirements.
An example would be SAE 10W.
Multi-viscosity oil or multi-weight oil has the
operating characteristics of a thin, light oil when cold
and a thicker, heavy oil when hot. A multi-weight oil is
numbered SAE 10W-30, 10W-40, 20W-50, and so on.
For example, a 10W-30 oil will flow easily (like 10W
oil) when starting a cold engine. It will then act as a
thicker oil (like 30 weight) when the engine warms to
operating temperature. This will make the engine start
more easily in cold weather. It will also provide
adequate film strength (thickness) when the engine is at
full operating temperature.
Normally, you should use the oil viscosity
recommended by the manufacturer, However, in a very
cold, high mileage, worn engine, higher viscosity may
be beneficial. Thicker oil will tend to seal the rings and
provide better bearing protection. It may also help cut
engine oil consumption and smoking.