I would just mention, if a fluid change does bugger all, how come ZF are recommending a fluid change at around 60,000 miles, I fully accept that a fluid change won't save a failing transmission, but it will prolong the life of a working normally transmission, in the same way an engine oil change will usually prolong its life, OK, so no combustion by products will mix with transmission fluid, the only thing to degrade a transmission oil is heat, metal, and clutch friction particles. The older mineral fluid will varnish over time, and becomes sticky, as mentioned synthetic transmission fluid is serious stuff, and will last almost indefinitely, but will still come out blackened when the fluid is changed, proving that it will degrade over time, and miles.