X3 on the 600wt. I got it at NAPA also. Grease will eventually cavitate if that's a word...gets pushed out away from the moving parts and stays there.
NAPA has it on the shelf. I just went in and told him I wanted the heaviest gear oil available, and that's what he sold me.
Look for 0 or 00 grade grease, it flows back in on itself. Regular gun grease will not flow back once its pushed to the side. A really good and cheap grease is John Deere corn head grease. Its a self leveling grease ( 0 grade ) available from any John Deere tractor supply. Its around $2 to $3 a tube and one usually will fill a gear box. http://www.greenpartstore.com/John-Deere-Special-Purpose-Corn-Head-Gun-Grease-AN102562.html http://lubromation.com/pdf/CONSISTENCYGRADES.pdf