API CD Class
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Engine oils for high-output diesel engines used in agricultural equipment (obsolete class).
The class was introduced in 1955 for ordinary use in some diesel engines, both naturally aspirated and turbocharged, with increased cylinder compression, where effective protection against deposits and wear was essential. Engine oils of this class could be used where the engine manufacturer imposed no additional fuel-quality requirements (including fuel with a raised sulphur content).
API CD automotive oils had to provide, compared with the earlier classes, better protection against bearing corrosion and high-temperature deposits in diesel engines. Engine oils of this class were often called "Caterpillar Series 3" because they met the certification requirements of Superior Lubricants (Series 3), developed by the tractor manufacturer Caterpillar.