There are no vacuum points on the E4OD. It is all electronic.  Unless yours has
been rebuilt with the updates, it is likely to have lost the fluid out the front
seal in which case, it drains right out the drip hole at the bottom of the
bellhousing.  Some of those transmissions had a habit of dumpin the fluid past
the front seal when they get hot because of an undersized drain from the pump
bushing back to the pan.  This allows fluid pressure just behind the front seal
to build up and push fluid past the seal.  Ususlly, the ones that did it may not
ever do it again.  Then again, once the seal has been breached, it may become a
regular habit.  The only fix I know for it is removal of the front pump and
drilling the drain back hole larger.  Of course, once you get that far with many
miles at all, you may as well rebuild the trans and do all the other updates.
In it's last version, it was really a very good transmission and the updates are
relatively cheap and easy to do during overhaul.

Lugnut

I would suggest that a 1990 vehicle might be low on fluid as a normal matter
of course. If you fill it and drive another 18 years before you fill it
again, then life is good and you have no worries. If there is a real
problem, you will be filling it again and again.

You made all of the checks that should be made, and found nothing out of
sorts. Drive around locally for a while with the new fluid, and after an
hour or two of driving, check for leaks, then after 10 or 15 hours, check
again, then after about 100 hours check again. If you have a problem worth
spending money on, it will become apparent. If you found that you lost some
fluid over 18 years of operation, then so be it.

For that vintage of E4ODs there was an occasional problem where
internal pressures would build up and the transmission would pump
fluid out the front seal. If you are lucky, the seal is not damaged,
and everything goes back to normal after the transmission cools. If
you are unlucky...not a good thought.

How do I know? I was lucky. I had a 1992 F150. I own a small farm. One
very hot day (think 100 degrees) I riding around in a plowed field
checking on a fire that was burning off some wheat straw in an
adjacent tract (even hotter temperatures around the fire, lots of stop
and go, and hard going because of the soft ground). I got out to check
on the fire and happened to notice a stream of transmission fluid
under the truck. My truck was still mobile, so I immediately drove to
my local repair shop. The mechanic was aware of the problem. He told
me to wait and let it cool off, add some fluid, and see what happens.
I did. 8 years and 60k miles later, the transmission was still working
OK. Maybe you will be lucky...

If not, a transmission repair is in your future. Think $600 to $1000.

Ed

UPDATE:
It's been 2weeks since this happened, andI have NOT had to add any
more fluid, though I've put about 350 miles
on it, most on the highway at 60 mph.

Thanks for the help, guys.
I'm keeping a gallon of fluid and the tranny funnel with me just in
case!

Tom