Transmission Fluid (Oil)

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The automatic transmission provides pressure regulated hydraulic fluid which is filtered for all of the transmissions functional requirements. See Fig 1. All BMW automatic transmissions are designed to operate with specific fluids. Use of non-approved oil will cause malfunctions and irreparable transmission damage which is not covered by BMW warranty. The transmission fluid provides the following functions:

Lubricates mechanical components (planetary gears, bearings etc.).
Removes heat and transfers heat to transmission cooling system. (heat exchanger).
Removes debris and contaminants to sump and filter when circulated.
Provides a transfer of kinetic energy in the torque converter.
Allows hydraulic operation of mechanical components (clutches, brakes) via control of the valve body.

Also, transmission fluid has various properties to prevent oxidation and breakdown from heat and friction. Each type of transmission fluid has properties specific for each transmission application. Fluid level is crucial in the proper operation of an automatic transmission. Improper fluid levels will cause improper operation and eventually irreparable transmission damage. Improper fluid level can cause:

A low fluid level can cause an interruption in oil flow during fast acceleration or hard braking which can cause gear shift malfunctions.
An excessively high fluid level can cause the rotating mechanical components to paddle in the oil. This produces foam which introduces air into the hydraulic system.
A low fluid level can also cause transmission overheating causing premature transmission failure.

Fig 1: Identifying Transmission Hydraulics
Courtesy of BMW OF NORTH AMERICA, INC.