The corn head grease is exactly the type needed, and it is called liquid grease, or fluid grease, designated as "00" and is available from several manufactures. Castrol make a good one.
This is a synthetic version of the 00 semi-liquid grease which lasts virtually forever. Doesn't break down from heat nor does it emulsify with water.
Packing the closed hub with regular grease does keep it from leaking out, but the grease is pushed away where the U-joint rotates so there is no on-going lubrication of the joint. The semi-liquid characteristic of the 00 lube allows it to pool around the U-joint seeping into the joint needles when the vehicle is sitting. BTW, if you install new U-Joints that happen to have the rubber lip seals around the edge of the needle bearing cup, remove those seals.
In an open knuckle those U-joint cup seals keep dirt out. In a closed knuckle they keep grease out. From experience I have found that the joints that have the seals removed so they get re-lubed by the pooling fluid grease, last longer than the U-joints do if the bearing cup seals are left in place. The churning of the joint rotates actually liquefies the grease around it letting the bearings to get grease. The liquification when churned is a characteristic called "thixotrophic."
Over the years I have used several brands of 00 semi liquid grease and they don't tend to leak even if the ball wiper seals are in poor shape.