This is from a F Series Operators manual Printed July 19, 1949. Maintenance fan- 5,000 miles. Turn fan until plug is at the top. Remove plug and add 1oz. *ENGINE OIL. Turn fan until hole is at bottom to drain excess oil. Install Plug.
Inside the fan is tube around the plug that will only leave a certain amount of oil inside the fan, when the plug is the down position. There is 2 gaskets that are $2 a piece. Can send pictures if interested. Just rebuilt one for my truck.