Transmission fluid needs to meets certain criteria that plain hydraulic fluid cannot - mainly it needs to allow "grip" of the friction elements.

There may be other characteristics different between the two fluids:
- temperature operating range
- detergency
- dirpersants
- antifoaming
- shear resistance
- viscosity improvers

Transmission fluid is highly engineered. Even among the hydraulic fluids there are different grades depending on the requirements of the end user. There's no way I would put in unapproved hydraulic fluid in my transmission.

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95 E320 Cabriolet, 169K