While mowing the lawn the other day, I started thinking about my next oil change this fall and what I wanted to do. I know it's odd to think about cold weather oil in July, but it helped to pass the time... I've got a Briggs opposed twin in my tractor. I run Rotella 15w40 during the summer, and had been changing over to Rotella 5w40 in the fall. I'm considering going to the 5w40 year round, but it can be a bit tricky to find around here. Which leads me to consider the 15w40 as the year-round choice. And I know that will raise a few eyebrows... My tractor stays in a warm garage all winter. I get that lighter weights are recommended for starting in cool temps, but this thing is starting in 40-45 degree temperatures. Even a straight 30 weight might be OK for starting in that temp, according to Briggs' online oil charts. Not optimal, but OK. Obviously, it's a lot colder outside when I'm snowblowing but at that point the engine's running and warm. Any meaningful snowfall seems to work the engine pretty good, so it's not idling along by any means. My fundamental question here is this. Should one make an oil choice based on the temperature the motor is when it's started? Or the ambient temperature it's operated in? I'm leaning to the starting temperature, but would appreciate opinions.