Just to set the record straight 5W50 Oil & Hydraulic Oil or Fluid are not the same or neither can they be interchanged. My Surfer calls for 5W50. I had searched numerous auto parts stores, mower dealers, the usual spots you might think this would be. Finally I stopped in to our local NAPA store & asked them what I could use in it's place since I couldnt find 5W50 oil. Use hydaulic fluid & you will be fine. I did this early spring & I had been running it for about 3 months. I've noticed that I couldnt climb hills as well as last season. Needless to say my dealer has my mower draining right now, flushing out the system. Luckily he found 5W50 about 30 miles away, I've asked them to stock this for me. I hope & pray I havent ruined the seals or damaged any valves. Lesson learned: Use what the manufacturer recommends!