Engine oil type for Cub Cadet Z-Force Engine?

I have been doing routine maintenance on my 48" Cub Cadet Z-Force Engine w/ the Fab. Deck. I know the Hydraulic Fluid is 15-40, but I can not see anywhere where it states what the Engine Oil should be, I changed it with the Kohler 10-30, but someone mentioned that it seemed like a thin oil to use.