Re the auto box, changing the oil is very worthwhile
BUT YOU MUST USE JWS3309 SPEC OIL AND NOTHING ELSE
Couple of videos showing each but the first is on a 5 speed auto and the second on the later 6 speed auto.
5 speed you refill via the dipstick tube on gearbox.
You have 2 options.
1. Sump Dumps which is where you remove the gearbox drainplug and release the sump contents ( only about 3 ltrs will come out and a further 4 will remain in the torque convertor) . Fill back up with fresh fluid to the correct level and then after a couple of hundred miles repeat the process and keep doing it for about another 4 times.
Yes you are mixing new oil with old BUT it is gentle on the gearbox and eventually you end up with fresh fluid.
Think of it this way, if you poured a glass of orange juice and drank half and then topped up with water and then drank half and topped up with water, eventually you would be drinking just water.
2 The Gibbons method.