Hello everyone.
After checking several cars to own to replace my previous one, and seeing an "approval" from Scotty, I recently owned a Jaguar S-Type that I love it. Everything runs fine in it, except one thing (and I might suspect why it happens).

The car is a Jaguar S-Type from 2002, one of the first phase II of the car (with reworked interior and new gearbox)
Engine is petrol 2.5 V6 (AJ-V6, Jaguar variant of Ford Duratec)
Gearbox is an automatic ZF 6HP26
Car is 145 000 km (90000 miles)
Last service been made by the previous owner in dealership at 135 000 km.

So I bought this car from a couple who knows live in a eldercare and didn't used the car anymore, and they lived in Brussels. The car was used basically for small runs, just to go to the supermarket for example. Good deal, works like a charm, except one thing I noticed once I drove it back at home (probably never seen an highway since years).
Anyway, I noticed that the 6th gear often (not always, but very often) pass quite roughly. No iron or weird sound, but not smooth either (like you drive on a small hole). 
Only does on the 6th gear and only from 5 to 6, never from 6 to 5. No issue on the other gears.
I have to check if the issue happens in Sport mode too, as it modifies how the gearbox works (I almost never use it)

I read that the gearbox needs to be serviced every 60 000 km, and the car is every 15 000 km. I didn't made any work yet, looking on the web about the issue. So I'm wondering if just changing the oil is enough, or if I have to worry about a potential trouble on this gearbox.

Looking forward to hear you, thanks in advance for your advices. Cheers
Lunya from Belgium.