I have read about 8 ounces. I just filled mine to the fill plug with a 140 wt. heavy oil from chevron. Really thick heavy stuff but not grease. It will run if given enough time. Many say you can also use John Deere corn-head grease. Chassis grease is not a good option as it pushes away from the moving parts making it about useless. I suppose the heavy oil will be slung up on the upper king pin bearing so it does not fail.
http://www.greenpartstore.com/John-Deere-Special-Purpose-Corn-Head-Gun-Grease-AN102562.html