If your tranny was severely under filled more than likely you would have experienced some slippage, and a long delay when shifting from P to D and D to R, Ect. If it was overfilled you may see some fluid coming out of the vent hose, and some crazy shifting like skipping a gear or not wanting to go into 1st from a dead stop. So if that tranny has never been overheated you should be ok. Second thing, try not to drive around after you have replaced the filter and added the new oil, nothing is going to get hurt by just letting the truck warm up in the driveway. Just let the truck sit at a idle for 20min or so, and run it threw the gears every few minutes. another good way to warm the tranny a little faster is to, and only if you trust your e-brake and wedge a tire, place tranny in Drive(D) and wait 15min or so, then you should only have to run through the gears one time just before you shut off the motor, this is the method I use. and Third, that tranny holds around 10.5 qts, about 5 of those qts are in the torque converter, cooling lines, between the clutches, ect, and those 5 qts will remain in the torque converter, and cooling lines, ect. When you drop the pan and change the filter there can be as much as 1/2 qt in the filter membrane and the other 5 qts are in the pan. So a good rule of thumb is to add 1/2 of the (total capacity)recommended amount listed in your repair manual. save about half of the last quart so you can add (only if needed) after you have warmed it up, and checked your dip stick. I hope this clears things up for you, and makes your next tranny servicing more understanding.