keeping OD off and staying in 3rd just creates more heat for the tranny due to it spinning at faster rpm's. back when i was on TCCoA there were numerous discussions and it seemed the people who drove around with OD off had on average, more tranny issues or rebuilds than those that just left it on. I was one that turned it off and went through 2 fresh rebuilds before finally working my 3rd 4r70w over (tranny cooler, j-mod, 3k stall converter, etc). That final tranny lasted a LONG time, i think 4 years almost under my sisters driving (which is multiple times more crazy than mine).
the valve bodies in the Mustang's 4r70w are considerably better but if the tranny does not have one already, a tranny cooler is a cheap investement that will prolong the lifespan of the tranny under any driving conditions.
1998 Eclipse GST Spyder - 14b turbo | 3" Catback | Evo8 BoV | 170fwhp if that
2003 Redfire Cobra - 448whp/435wtq - Sold