Grease is the answer, John Deere corn head grease, self leveling, flows back on itself yet will not leak out of the seals. $3.00 per tube at most John Deere service centers or online. One tube will do a gear box. Axle grease will only get pushed out of the gears and not return, thus after awhile the gears run dry. 1/2 gear oil, 1/2 axle grease is a option.
Joe
