Hello I own a 2006 XJ8 with 137,000 kms on the clock.  Apparently this has a lifetime transmission fluid, I believe that nothing is lifetime.  Has anyone had experience flushing their transmission fluid, were there any problems that resulted after the service?  What is the general consensus on transmission service? 

With cars above 60000 miles (~100000km) i've always adopted the "part-change" method or as some call it, the "sump dump" method.

Undo the drain plug after a 5-8 mile run and drain the contents of the sump. Refit the drain plug and top up with fresh fluid to the correct level. Repeat 3 times at 1-200 mile intervals and then do one drain/refill operation a year to keep it fresh.

This means you are adding the new, thicker fluid gently and with it, the seal conditioner present in all ATFs. The conditioner will have worn away in the old fluid, potentially allowing the seals to become dry and brittle and if all the fluid is changed in one go (like in the flushing or "Gibbon" method) then it suddenly gets all new, thicker fluid with the associated increases in pressure. This can cause rupture of seals that are dry/brittle due to the extra pressure before the seal conditioner has time to do its magic. Also this way of doing 4 part-changes means any dirt has 4 chances of being drained, further cleaning the inside of the box.

I also subscribe that no fluid lasts the lifetime with the theory that once it fails, so does the unit it lubes - to that end it has lasted the lifetime of the unit but that lifetime could be so much longer if the fluid is regularly changed.