You bring your car to the oil-change place and they slap a sticker on the windshield: Change it again in 3,000 miles or three months. You probably knew that the 3,000-mile interval was ridiculous, but what about that other deadline? What if you don’t drive much and let your oil sit for six months? A year? I know a guy with a fleet of Ferraris, some of them seven-­figure cars, and he goes only by mileage. “That oil was in the ground for 100 million years,” he says. “What’s six more months?” Good question, one that I recently answered through semi-intentional negligence. Here’s what happened: In 2008, I bought a Troy-Bilt lawnmower with a Honda engine. After a few years, I realized I hadn’t changed the oil. But, by then, electric mowers were getting cheap, and I really wanted an electric mower. I figured I’d just let the Troy-Bilt go until it blew up. Which it refused to do. I kept mowing my lawn, season after season, the little Honda purring away, until I eventually started feeling bad for the thing. I decided that if I pulled the trigger on an electric mower, I could at least give my Troy-Bilt to someone who would use it. I warmed it up, tilted it over, and drained the oil, savi...