>drain the oil completely and then refill the tranny with new oil.  My
>manual says to use SAE 90 hypoid oil.  First of all, is this something I

Yes, and do the reduction boxes, too. 

YOu'll need the 17mm hex tool to get the filler and drain plugs off.  Remove
the filler
plug first, so you know that you can.

I'm using 85/140 in my bus.

Thom Fitzpatrick
thom@calweb.com - http://www.calweb.com/~thom
Keeper of the Barndoor Bus Web Page: http://www.calweb.com/~thom/barndoor.html