John, this motor is a little different from the later motors in the mid to late 1950’s. That gasket between the gearcase and the mid-section is critical to seal in the gear oil. Yes, it is open in the back for venting purposes, but the closed front area separates the gear oil from the surrounding water. The gear oil vent screw is above the mating surface of the gasket, so gear oil is present above this gasket also.
It sounds like what you most likely need is a NOS gasket for the gearcase/mid-section mating surface. If I remember correctly, the seal area is separated from the gear oil, so this is why I think it’s most likely the gasket.
I made one of these gaskets once from fiber-rubber gasket material, and it failed when the vapor pressure peaked on a hot summer day. Maybe cork sheeting would work, but the original NOS gaskets definitely work.
Regarding the gear oil, 90W is preferred, but John Deere corn head grease or Lubriplate is ok. This has modern seals, so that’s why gear oil is recommended.
If you end up replacing the gasket, Permatex Ultra Black is hard to beat for a sealant for this application.
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