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“What kind of gear oil should I use in the transmission?”
Originally, the 8N used 90w (GL-1) gear oil for the transmission, rear gears and hydraulics common sump. Capacity is 5 gallons. Many people today recommend using Ford 134 oil. This usually works fine in the 8N and is not so stiff in cold weather. However, if your system tends to leak, or you have a lot of wear in the hydraulic system, you probably need the heavier oil to keep things working and slow the leakdown rate. I use 80w90 gear oil (GL-5) or 90w (GL-1) in mine, just because it always works fine for me. When draining the gear oil, be sure to remove all 3 drain plugs (one under the transmission, one under the hydraulic pump, and a 3/4″ pipe plug under the rear differential).
Thanks to John Smith of Old Ford Tractor for allowing us to use this information.