Listen I know this has been gone over a million times. I went into my local shop where BTW I've purchased every one of my mowers, and I negatively commented on somebody who came in and bought a quart of "Honda" oil. Both the General Manager and the Lead salesman told me I should also be buying the "Kohler" oil for use in my Toro's. They both stated that the air cooled engines run considerably hotter than a water cooled engine. Blah, blah, blah. Said I could be doing some considerable damage to my engines. I've never seen anything in any manual that suggested I was required to use anything other than regular motor oil. I change oil and filter every 50 hours or sooner. Every time I've ever changed oil it's always been fairly clean and sludge free and with no offensive odor.... What's the general consensus?