There are three drain plugs for the N hydraulic system. Often folks only loosen one or two and leave the third one, on the diffy case, neglected. This is one reason systems get overfilled and leaks occur. When draining the hydraulics system, best to get the oil warmed up/hot so run the tractor for a good 20-30 minutes first. Loosen one drain plug at a time and always start with the diffy case plug and move forward towards the front of tractor. The diffy plug is an NPT (pipe) plug with a 9/16" square head. This plug can be stubborn if it has not been removed in many years, if ever at all. You may need to use a good Crescent wrench with a cheater bar. Next, move to the Hydraulic System drain plug. You'll need a good 1-1/16" hex wrench or socket, and possibly a breaker bar. Try not to let the plug drop into your drip pan. Note or market so you know which drain hole it goes back on to. The third plug is the Transmission Drain plug and it too uses the 1-1/16" hex wrench. Once oil is drained, replace plugs. Wipe off excess oil dry on plugs and sealing surfaces. The diffy plug has no gasket but I always put a very small dab of Anti-Seize on it. The two large plugs have gaskets. Originals were cork but sold today as paper or rubber. I take some clean new hydraulic oil with my clean finger and wipe the new gasket both sides then seat onto the plug OD before inserting into drain hole. When tightening these plugs, just use the wrench and no breaker bar - do not overtighten. Use new oil -never reuse old oil as it could be contaminated with water. You can do an archive search here to read about hydraulic oil and what to use. The original Ford spec'd MC-134D oil is no longer supplied by CNH (Case/New Holland) but there are other solutions. The TSC oil is a cheap, sufficient product for most applications. In colder climates, you may want to consider other alternatives. The hydraulic system capacity is 5 gallons. NEVER pour that much into your system at one time. Even if you do a system flush and cleaning, there is always oil in the system. When adding the new oil, only put 3 to 4 gallons in at first. Run the tractor, get oil circulated good, and check for leaks. Shut down, let settle for a few hours, then check level again. Add a bit more if needed but you may be surprised to find what you already put in is good.