Finally the transmission servicing is complete, after fitting a new filter, seal, fluid pan + drain plug and 4 fluid changes.

As suggested elevating the front right wheel only during the drain yielded 4.5L (4.8Q) i.e. 10% more than any for other combination..

The fluid "purity" after 4 changes, I estimate to be approx 90% (89%) but of course 100% is not possible without flushing.

As for transmission performance (if that's the correct term) gear changes seem a lot smoother and quieter, especially travelling up inclines.

The results were very positive, it 100% needed doing and I'd like to off a big "Thank You" to all those who helped ...

 

Yep, I have flushed all my trans every 25k. Its very easy to flush the hummer just disconnect the the top tranny line on the radiator and hook up a hose to a bucket for the future.