Most of those MF65's use 80- 90 Wt gear oil for hydraulic fluid... If you put anything with a lighter viscosity you will have the exact problem you are having until you drain and replace the fluid. Also when changing the fluid there is an internal filter that can get clogged up, remove it and wash it with diesel or kero... While at it do not over look the final drives, they have planetary drives on each axle that must be drained and filled. (will not drain with hydraulic system) MF65 is a wonderful utility tractor. KennyV