New driver

i just got a 98 windstar with overdrive and i was wondering if i should put it in overdrive all the time or is there certain times to put it in OD or regular drive? and will keeping it in overdrive cause the transmission to slip? thanks.

You should normally leave the overdrive on. I'll quote directly from my Windstar manual...



"When to use D (Drive) - with the Overdrive Off button activated

You will notice that there is only one drive position on your
gearshift indicator (instead of Drive and Overdrive). However,
you will find a button and an indicator light located on the
gearshift lever. Push in the button on the end of the lever and
the OVERDRIVE OFF light on the end of the gearshift lever will
illuminate. With the OVERDRIVE OFF light illuminated, the
transaxle will operate in first, second and third gears and will
not shift into fourth gear. Operating in Drive (OVERDRIVE OFF
light illuminated) provides more engine braking than Overdrive
for descending hills or city driving.

To return the transaxle to the normal Overdrive operation, press
the OVERDRIVE ON/OFF button again. Use this button to
select Overdrive or Overdrive Off whenever you drive your
vehicle.
The transaxle will be in the “overdrive on” mode when the
vehicle is started even if the O/D OFF mode was selected when
the vehicle was last shut off.
NOTE: If the O/D OFF indicator light is flashing on and off
repeatedly when the vehicle is started, there is a
transaxle electronic system malfunction. Contact your
dealer as soon as possible."

So I always leave mine in overdrive, and I suggest you do the same unless you have some reason not to.