I just purchased 8n13325 and started restoring to working condition.
just did the same thing. Changed the hydraulic/transmission fluid. Oil has changed and iso numbers and viscosity are 2 different methods of usage. I use a full synthetic oil made by Amsoil that is 10W-30.
My 45 years with Amsoil, I did a complete gasket swap from filler plug to drain plug. Axle seals to inspection covers.
VP makes a low priced oil sold at Tractor Supply Co. TSC has an app.