the only difference between a m37 and m715 np200 t-case is the speedo meter drive gear & cable hookup drive assy. plus the mechanical parking brake levers on the m37 are totally different so the m715 linkage is too short plus the m37 levers will hit the bottom of the cab when you try to apply the parking brake. you can remove all the incorrect parts from the m37 case and remove & install the m715 parts on the m37 case and all is good. i learned all this the hard way by r/r-ing the t-case 3 times by myself. the cases are identicle other than these 2 points. i found a new old stock m37 t-case and bought it for my m715 is how i learned this stuff. nothing like new........... i run 80w140 castroil full synthetic in mine but i mostly drive to & from the sand dunes and ride in the snow around here.