Okay, so I meant to change the hydraulic oil at the recommended 50-hr mark, but slid it out to the 62-hr mark. Looked fine when changed, bluish green color, saw a few "copper looking" metal flakes, but fairly clean. Installed OEM filter ($$), filled with Mobil-1 15W-50 motor oil, used approximately 10-qts. I might need to add a bit more, but not much, it is close to the estimated 1-in from the top level. If the tank itself holds 3-gallons (12-qts), the hydraulic oil filter evidently displaces 2-qts of that total? So it seems too that the oil in the wheel motors and maybe the aluminum air shuttle cooler may still be the original oil, meaning I have some standard oil mixed with mostly synthetic oil. I guess this okay, but concerns me a bit as I am not sure how much of the old oil remains in the system. I also started it and let it run at 2/3 power for a few minutes, then engaged the drive, and it sounded "dry", but settled to it's normal whine after a few turns, forward and reverse maneuvers. Since it seemed fine, I mowed for 2-hrs after this change without issue. While some noted a marked decrease in whine after the first change, my hydraulic whine seems about the same as before. Did I do this right or no?