Joe Black does the best job of explaining the service here:
http://www.antiquecaterpillar.org/vx...l/4L30E_01.asp

The service CD has another procedure for checking fluid, and it's not likely to be known by the average Jiffy mechanic.

1.) Park vehicle on level ground, apply parking brake.
2.) Check fluid level with engine running at idle
NOTE: Ensure transmission fluid temperature is BELOW 86*F
3.) shift through all gear ranges
4.) shift into park
5.) idle for 3 minutes, then open the overfill screw(the top one).
6.) add fluid until it flows out of the opening
7.) let engine idle until a fluid temp. between 90*F and 135*F is reached, THEN close the overfill screw. Fluid temp is checked with a scan tool.
8.) torque screw to 28 lb ft.

Minimum fluid level @135*F
Maximum fluid level @ 90*F

DO NOT:
-open the overfill screw with the engine stopped
-immediately after driving at sustained highway speeds
-in city traffic, during hot weather
The vehicle should be allowed to "cool" for 30 minutes if any of these conditions are present.

I should have made my earlier post clearer -it's easy to assume that the fluid level is full when then engine is off. Most people (myself included) are not too keen on sticking their finger into a fill hole when the engine is running.
If the engine is hot, they could've figured that it was low on fluid then added some to bring the level up -overfilling it. This would cause driveability issues as well.