Okay, I'm working on my '66 Westy which hasn't been driven in 13 years.
I'm replacing the axle boots because they're crumbly and one is actually
leaking oil.  I have both of the new boots and I'm wondering if I should
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
can get at my FLAPS? and secondly, can I refill it myself?  The engine is
out right now so I can probably get to the fill plug from above.--

with the engine out is the easiest time to change the gear oil.   buy a
gallon of 80/90 weight gear oil, and the pump.  prolly ~$15 for both.  you'll
also need a 17mm allen wrench for the trans oil plugs.  there are two drain
plugs in the main section, and oil fill plug.  you should think about doing
the gear boxes too, for this youll have to pull the rim off to get to the
fill plug, and to catch the oil.

if the old oil is gunky  i'd change it agin in 1000 miles, and add the sick
50 tans goo

my two cents