do a test with thick tissue paper - suck out some ATF and dip the tissue in - put it aside, dip the tissue in the radiator coolant and put aside - now suck out some engine oil and dip a tissue in it and put aside, which look similar?
if you had gearbox trouble then connect a star scan on it and read the errors, If you have history of the conductor plate being changed and you have a pending code of speed sensor fault in or out - then replace the plate and fill with fresh ATF.
Also remove the TCU from the "server room" in the engine compartment and check for ATF in there, if you find some ATF in there then buy 3 cans of contact cleaner and wash it out by opening up the TCU plastic shell. Just hope this "gear problem" was not due to the driver spilling chai/water in the gearshift lever, if he has then kiss it goodbye, you need a new/used/refurb one.