I wouldn't be too quick to downplay the importance of manufactures' recommended lubes because it's 'just a gearbox'.
Just as with an engine, often a range of viscositys are acceptable depending on the operating temperatures. 80W-90 will flow like SAE80 while cold and like SAE90 once warm and should be fine in the gearbox, 80W-140 however is too thick... just a little FYI..
I too use the 80W-90 for the MMM gearbox. Just remember that gearbox's input shaft is turning at 2500RPM, too thick of oil will build up excessive heat fast!
Thanks for all your feedback, guys! I stopped by O'Reilly's today and picked up some 80W-90. In general, I try to treat the manufacturer's specification (for any kind of machine/vehicle/appliance) with the utmost respect (I'm a design engineer, too). I probably get a little too anal sometimes.