da numbers simply refer to safe operating temps. Colder temps need lower 1st #(looser in the cold), hotter temps need higher 2nd number(more durable in the heat). Usually, you will run a different oil for winter and summer.
Too high a frist number in the winter will result in poor circulation upon starting and can lead to premature wear or more immediate short term damage. ( hard lesson learned durning a winter @ 10,000 ft)
Too low a 2nd number in the summer can lead to premature breakdown from the heat and eventual damage.