S-Type auto gear box oil replacement & diiferent oils to use

Hello Dear Members,

I have 2007 S-Type Jag with 94K miles 2.7L V6 se diesel , I have recently purchased it around 8 months ago.

I have had conflicting data relating to the "sealed for life gear box oil" some are saying, change it after 60K miles, some are saying a leave it a bit longer 80K etc.  Am I correct to assume its not a "sealed for life gear box", as the colour of the oil is now a very dark brown colour, and not a very light brown, I purchased a litre of jaguar ATF replacement oil to compare colours etc, (after picking myself off the floor) £25 litre!!

My question is, can I use an alternative oil, e.g. ATF Dexron 3  or 4 by millers or by Mobil?

or the only oil I should use is the very expensive Jaguar own mineral oil?

Also is it possible to do this as a DIYer or should it be for the professionals,

is the capacity 6.6 litres of ATF oil for a maintenance replacement for my car?

Can the gearbox oil pan and oil filter be washed out and reused?

thank you in anticipation.

Roy,