Though the manual doesn't specify a maintenance schedule, I don't think it would hurt to change your MTX fluid at 90k; I know some ppl who change it every 50k or so. I switched over to the synthetic ATF/friction modifier "cocktail" at 58k 3 years ago and am thinking it's time to put in some fresh fluid (I'm at 110k now).

I would caution against using gear oil - it apparently makes the shifting very stiff. See:

http://www.coldair.org/archives/9912/msg00664.html

Here are some wise words from Ray McNairy (may he Rest In Peace) on the subject:

http://www.coldair.org/archives/9912/msg00670.html
http://www.coldair.org/archives/9811/msg00568.html

And some additional info/ponderings:

http://www.coldair.org/archives/9805/msg00233.html
http://www.coldair.org/archives/9911/msg00557.html

I personally would not want to risk ruining my synchros and would go with what Ford recommends.

'98 GL V6/MTX (Plenty of mods... in my basement) '88 ST165 (mild mods, stock 8psi)