And back in to the shed he goes

I have been looking in to the Dextron III / VI mixing, and from what I could find out Dextron VI might have an adverse reaction to seals in the box, IF your car is pre 2006 and original seals are still in there. Apparently the 2 fluids will mix and not cause any problems other than the seals. After 2006 all seals were OK with fully synthetic oil

So if your box is still un-rebuilt I would be wary, as you might cause yourself some , if it's been rebuilt then it should be OK. BUT IT IS UP TO YOU, I AM ONLY REPORTING ON WHAT I READ ON THE WEB.

Personally, I'm going to stick with Texaco 7045E as everyone was very anal about that specific make and grade, and I went and got 20 ltrs to do my box, which I suspect has DextronVI in at the moment. But having found that info, unless it's a really nasty colour I'll just change the sump oil rather than attempting a full gearbox flush. Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!