Just to add to the mix, copy & pasted from my post on the thread linked to by Tony above :
Don't flush it! Not with nearly 300k miles on it!
Do a "part-change" instead, drain the sump, either by removing the drain plug or using an oil extraction pump (about £15 on ebay and no crawling underneath!) and refill to the correct level using new fluid. I use Carlube ATF-U and have found it's an excellent fluid and suited to the Aisin-Warner box very well.
The first part-change will probably give you an instant improvement, do another in about a months time then another about a month later. This will in effect change all the fluid very gently and get a lot of dirt/contamination out of the box while reducing the risk of blowing seals with new, much thicker, fluid.
It also gives the seal conditioner present in the new fluid time to work which will help the seals much more than flushing it through with umpteen litres of new, thick fluid that could easily blow a weak seal to oblivion.
After the 3 part-changes, do one part-change a year to maintain the service changes at the right interval.
If it's not been done for a while, i'd suggest the gentle approach (part changes) to be on the safe side. I know others have flushed their boxes but there's always one where it goes pear-shaped.
