My 97 xk8 have 34,000 miles and is running perfect. I bought it september last year with 31,500 miles and immediately worked out the tensioners, thermostat, shock mounts, power steering hose/oil cooler and some other minor things here and there.

The only major not done yet is the change of the atf/filter, which by the way, is working perfect

when, at what time/mileage is the most proper time to do it?

 

Jaguar specified that the ATF was 'lifetime' which has proved to mean until the transmission failed. ZF now recommends a 60k change interval on their rebuilt transmissions.

The most significant internal failure has turned out to the physical failure of the 'A' drum which has little or nothing to do with the ATF.

Lots of discussions on the subject and sources of ZF spec fluid or fluid that specify meeting the specs in the FAQ sticky above.

 

Agree with TestPoint. I think you're safe with ATF until at least 60K.

Some BMW's have the same ZF_5HP24 trans as our '97. Originally BMW said it was a lifetime ATF fill; now they indicate 100K is the correct mileage for a replacement fill. Consequently I think everyone should change their trans fluid sometime between 60 and 100k; with sooner being better than later.

ZF 5HP24 transmission - Wikipedia, the free encyclopedia