I guess that you have not been doing the partial changes called out in the maintenance book.

That is, drain the pan, top up with fresh fluid.

In the AWD board, some owners are no longer doing a full change because of horror stories being submitted by people who have had transmission problems after a full flush.

They have gone to a - slow flush - by dropping the fluid in the pan and topping off as was done with 7 series cars.

This can be done at shorter intervals than the book, such as every 10,000 miles.

I have read of other manufacturers doing the same thing.

Have a look here for the detailed procedure.
https://www.brickboard.com/FAQ/700-900/TransmissionAuto.htm#7XX940FluidFlush

I've done a few flushes using (sort of) this procedure and found that it works well. If there is no drain plug on your trans, just eliminate that step and go ahead with the flush using the radiator/trans cooler fitting.

I have done it with no helper to add as I drain. You must stop the engine as soon as you see bubbles coming into the clear plastic hose from the rad fitting. Take note of the amount that drained out in that step and replace that same amount into the dipstick tube. I recall you will get about 2 litres before the bubbles start appearing in each step. Repeat the steps until clear, red fluid appears in the plastic tube. Check level using the correct marks on the dipstick for the trans temperature you think you have...probably the cool temp at this point.

Go for a 20-minute drive to get to full temperature and check again. Hopefully it's correct or just down a little, as it can be a bear to drain just the right amount. I deliberately leave it a little short on that last step, and top up later.

Mobil-1 synthetic ATF would be fine for your trans but a close second and significantly cheaper is the newest Dexron specification: Dexron VI. I use Valvoline "Max Life". It will take about 10-12 litres for the complete flush. I would avoid the "power flush" that repair shops sell...heard a few ugly stories.
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Bob: Son's XC70, my 83 244DL, 89 745 (V8), 98 S90, and XC60. Also '77 MGB and some old motorcycles

Flush it yourself undo the line at the radiator place some clear hose on it run the car in park as it pumps the fluid out into a bucket you can see the colour through the clear hose at the same time pour new fluid in the transmission until you see fresh fluid going into the bucket also shift it into neutral and through the gears to give it a good clean.

Full manual flush. IPD has a kit or you can manufacture one. Or just keep doing what you're doing. Drain what you can through the plug and refill, I assume?

It is my understanding that the power flush can be too hard on the tranny's internal parts. At 300k, I certainly wouldn't want to do anything too disruptive to the "old girls" parts...:)