328i gearbox smoke

A friend of mine has a 1998 328i auto. Friday last week, he was towing a caravan up a steep incline +-5km long in 3rd at 80km/h (3,500RPM). When he
started the decent, the car was engulfed in smoke coming from the transmission, or that's what it seemed like 'cause the smoke comes through the gearbox console. This happened again on Sunday when returning home. I drive the same car, manual with a heavier caravan and I had none of the symptoms.
Anyone with experience of this or any ideas as to what the cause might be.
Kind regards MW
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According to the local gearbox specialists this was caused by the previous owner replacing the BMW spec ATF with a cheap alternative. Apparently this causes the gearbox to overheat under strain and blow off some oil through the breather. A assume that it ends up on the exhaust causing all the smoke. Gearbox is now full of the right stuff and changes are much better also. Will see the next time he does that hillclimb if the gearbox still overheats.
Regards MW

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