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There are three screws on top of the steering box.
Normaly you remove the screw next to the driver and the screw next to the front of the car, press in the lube with the help of a press and wait until the lube is forced out through the other screwhole...
You should have a lithium based grease, sometimes called chassis lube, in your steering box. I do not recommend using oil in the box, because this will increase the chance of a leakage...
I can get you the number of the grease if you need it, but I´ll have to search for it..
Concerning your question, normaly you don´t have to change the lube, but for checking I recommend unscrewing one of the bolts on top of the gear box, BUT NOT THE ONE IN THE MIDDLE!!, this is a adjustment screw.
then use a screwdriver and ckeck out how much lube is in there.
Here is the part number GM #12377985. Go to any GM dealer and they should have it. This is a grease gun type tube so either you need a grease gun or some other way to get the grease in the steering box. Look for the post from a few weeks ago for more information on this subject.
John