Perhaps that's true.....I havent monitored temperatures whilst towing yet but they are only going to be worse than what I've already monitored and will be heading towards the 260 mark.

As far as I'm concerned, just like with engine oil - if I can get it to run cooler (obviously not too cool) then it will do it's job better than it would if it was hot. If I can get it down 15-20degrees then I think the box will be a whole lot happier (aswell as my peace of mind). I'll also be installing an inline oil thermostat aswell to ensure that it doesn't run too cool. I think BMW designed them to run them hot so that it burnt off any moisture in the fluid so that it would potentially last longer. I'd rather run the box cooler and change the fluid more regularly to be honest. I think the 6 speed boxes in the 4.8is run around 80-90degrees which I'd be much happier with.

Would you class the 5HP24 box as 'modern'? I'm not sure I would. It was first introduced in 1996 which is almost 20 years ago.