When was the last time the fluid was changed?
if you do a 1x fill, thats only 33% of the entire fluid, effectively you're not changing all that much.. and it turning brown? it's probably the new synthetic fluid doing it's job: cleaning.
a full replacement is like 4x fills, but people usually do 3x drain fills....
thats why you should do a drain fill every 2nd service to spread out the costs
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