Corn head grease is a 00 grade, which is self leveling. It flows back in on itself as it is used, very much like a supper thick gear oil. Any brand of grease will work if its NLGI 00 or NLGI 000 grade grease. Normal chassis grease will be pushed to the corners and stay there.
As for how much, enough to cover the gears in the box. My '41 gear box holds almost one whole tube of grease, your might be more since its newer and probably bigger. To much grease can be pushed out the input shaft and up the steering column tube.
John Deere grease can be bought online or at any dealer ship. The lawn mower dealers may have to order it in, and the price is around $3.00 a tube.
Joe
