Engine Oils
Background & Properties
castrol.com/GPS
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May 2025
GPS
e API Service Symbol
e API Service Symbol "Donut" is divided into three parts:
• e top half describes the oil's performance level.
• e center identifies the oil's viscosity.
• e bottom half tells whether the oil has demonstrated energy-conserving properties in a standard
test in
comparison to a reference oil.
Performance Levels
e top of the Donut shows the oil's performance level for gasoline and/or diesel engines. e letter "S"
followed by another letter (for example, SM) refers to oil suitable for certain gasoline engines. e letter
"C" followed by another letter and/or number (for example, CK-4) refers to oil suitable for certain diesel
engines. ese letters officially stand for "Service" and "Commercial."
SAE Viscosity Grade
e center of the Donut shows the oil's SAE viscosity grade. Viscosity is a measure of an oil's flow
characteristics, or thickness, at certain temperatures.
A multi-grade oil (for example, SAE 5W-30) provides good flow capability for cold weather but still
retains thickness for high-temperature lubrication.
A single grade oil (a single number in the center of the Donut) is recommended for use under a much
narrower set
of temperature conditions than multi-grade oils.
Operators should refer to their owner's manuals to select the proper viscosity oil for the ambient
temperature and operating conditions at which the equipment will be used.
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