You want the thicker oil for summer like 40 weight. I live in Chicago and the BMW service manager from two dealers don’t recommend 15W-50. They say it is too heavy for my car (1995 525i M50). I’m sure it not that much different from M30 engine. I would go with 0W-40. It is great all year round oil. I have it on my M50 engine and it runs better.