Proper lube for transmission

["Kenyon, Kirby T" <kenyonkt@maple.lemoyne.edu>]

I'm a proud new owner of an 80 Spitfire (last Friday) with over 200K
miles.

The previous owner was using the same oil in the transmission (4 Speed
with Overdrive) that he used in the engine (Castrol 20W50).  All the books
seem to indicate something more like 90W for the trans.  Does anybody have
any ideas as to why the lighter oil and the implications of using it?