I own an 8-N and a 1964 4000diesel I was advised by several folks as well as a ford tractor dealership mechanic who has his own tractor repair shop , He says 30 wt. motor oil in both the rear drive and trans on the 4000 and the 8-N will suffice , I use ford spec fluid from TSC in all of my tractors, the container says all ford tractors before (as I remember 1975 ), I was using 30 wt. oil before the switch to the TSC fluid, no problems so far 25 years and counting , I change my fluids every season.