W211 e320 cdi transmission fluid

[dieselbenz1]

Well last year after checking through the records the transmission fluid had never been changed. So I changed 3.5 L and the filter. At the time the fluid was black. Over the last year I drained the pan and replaced 3 l twice but the color was still a deep coffee color. Today I changed filters again replaced 3.5 l of fluid the removed the trany return line at the radiator and pushed 3 l at a time 3 times by running the engine and refilling the transmission. The third time the fluid came back cherry red. So I'll monitor this and see how long that color lasts.