L322 2008 SC ZF 6 speed oil flush?

Hi , I have a 2008 Supercharged 4.2, I've had it for a few years with no major problems. The gearbox can be a bit lumpy/hesitant on kickdown sometimes. Its done 135K and I'm not sure if it has every had an oil change/flush.

What would you suggest I do, is it worth flushing the gearbox, or changing the oil? I've heard conflicting stories that either can cause issues especially if not done correctly. If it ain't bust should I leave well alone?

Read this https://www.landyzone.co.uk/land-rover/the-zf-6hp26-service-procedure.341840/

These gearboxes need a service every 60k..

Sump dump with the cheapest properly specc'd oil, and then again in 500 miles whereby I'd advise a new filter, a proper genuine one.

thanks