Since Day 1 when I brought this Troybilt home 14 years ago, I've used Brad Penn 30w. First change was at 5 hours, then every 10 to 12 hours after that.
Brad Penn has ZDDP, which I'm told used to be in all oils, but they took it out to protect catalytic converters. It's often used in off-road engines, and when you think about it, a mower engine runs in about those same conditions: hot, dirty, high speeds, and high loads.
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I know she's not the fanciest or most expensive mower out there, but she's by far the best push mower I've ever had. And I'm confident Brad Penn oil has helped keep it that way.