I’m pretty sure you can mix synthetic and conventional without an issue. I put Fuchs Titan ATF 4400 Ultra High Performance in the tiptronic box. It works a treat, what ever that is worth.
Speak with Opie Oils - https://www.opieoils.co.uk an ex forum trader. They know their apples so will be able to advise you 100%. Lucas Transmission Fix (Stops Slip) is also a good addition when doing an oil change.
When refitting the pan make sure you use a small amount of gasket sealant (Loctite Grey SI 5660 to be precise) as you don’t want it going inside the pan. Make sure both surfaces are spotless. If the pan does leak (no reason why is should) don’t be tempted to fit a gasket (you can buy them) as it’s only a temp fix and will start to leak and then warp the mating faces. Careful not to over tighten the sump bolts, think they need 6 ftlb maybe a little more but not a lot.
If fitting a temp sensor for the box (very good idea btw) make sure it’s in the bottom of the sump, where the plug goes. You can also drill and tap for a sump magnet if you want to be super cautious. Nice mechanical gauges like stack are very accurate.