I have to ask just how bad is it to use the much more commonly available
SAE 80W-90 in there?  Or in other words, what is the benefit of these
harder-to-find oils?

Easier shifting, ok, got that.  Cooler temperatures?  Not clear if there is
any benefit from these alternatives - or is there?  Will they make the
transmission last another 200k?  Should I throw the giant jug of 80W-90
away or use it to flush the transmission?

Mostly I'm interested in knowing why this is a big deal.  It certainly
seems important to some - I just want in on why.  Is it just you want the
best of the best, or is it really that the common oil is crap.  What do I
get from better oil?

Thanks for any illumination.

Tom