If you have the ZF 5HP-18EH transmission, I don't believe you get it up to "operating" temperature. The fluid level has to be filled and checked between, I believe, about 20 to 45 degrees C (68 and 113 deg F) (may not be exact range). The point is, if you checked it at operating temperature, which is considerably higher, the fluid has expanded and has more volume - this may be why you had some fluid spill out of the fll plug when you opened it.