While changing my transmission oil I found out that I had been driving this '98 Windstar 3.8L van for 11 months (6K miles) with only 8 Qt of transmission oil (normal should be in the 12 - 13 Qt range). I never checked it before. I just noticed that the car shifts slower than normal and sometimes jerks hard when parking shifting between "D" and "R".

Now I filled everything back to normal levels. I'm bothered by the fact that I have been driving with insufficient oil for so long.

To what extent does it hurt the transmission? :screwy:

Was not good on it but only time will tell. If fluid is right now it may not show adverse effects. It may last as long as you own it or could go out one day. But just hope it will last.
Over heating of the trans and fluid may have been an adverse effect. Excess wear on internals would be another.

If you had to add 4 to 5 qts of fluid ... I'm a bit surprised it even operated at all ... as the pump usually cavitates at about 4 qts "low". That's the primary concern of having enough fluid ... to keep a solid flow of liquid going thru the pump as all the "upper" sections of the tranny get filled ... it kinda shuts itself down when the pump is empty. No pump pressure ... no nothing else.

So, since it did work, there's a good chance you had just enough fluid in it to prevent cavitation ... and no serious harm was done.

While changing my transmission oil ... only 8 Qt of transmission oil
If you dropped the pan, you didn't get all of it out. If you mean that you only had 8 quarts of fluid to recycle, there can still be quite a lot left in the system.

Hmmm - My first thought was "How did the original poster know it was 4 to 5 quarts low? If it's that low, the fluid won't even appear on the dipstick!"

So: How did you know it was that low? Did fluid appear on the dipstick? If so, how far below the "low" mark did it appear?

lapin_windstar is correct: Draining the transmission doesn't get the fluid that's within the torque converter (which is a fair amount), so you can't base fluid level on what comes out of the tranny.

The only accurate way is to check the dipstick, in the manner the owner's manual recommends - which (at minimum) generally includes a warm tranny with engine running, parked on a level surface.