Hi Torino, it is pretty much impossible to drain your entire machine of hydraulic oil when changing it. There will always be some left in the cylinders and lines. Many members choose to drain their oil by connecting an open ended hose to one of their aux connections and running the machine with the aux activated until it begins to sputter oil. Shut off the machine, change the filter and refill with Bobcat hydraulic fluid. As far as engine oil goes, it is a similar process to changing the oil in your car. Use a good quality diesel engine oil and a quality filter. HTH