I'd be more worried about filter intervals than fluid intervals.
It's like changing over to syn from dino on an engine - the engine ultimately won't care to have the superior syn in it. You're just going to have filter degradation running extended intervals. If we're speaking MB, synthetic oils use a fleece (polyester, etc) filter rather than paper filters found in these older cars. By the time you reach that higher mileage interval, your paper filter has broken apart or collapsed.
I have a few instances up on my blog:
W211: Please maintain your cars....... ~ Artisan
PreviousOwner: Oil Filter ~ Artisan
As to the original point of this thread - I honestly wouldn't be too worried running a trans 45K miles on synthetic. Just depends how long it sits in there, is all. 45k miles in 2-3 years is cool, any longer than that, I'd be changing out fluids/filters on principle. Who knows what's going on in there.