If the Mobil 1 0W-40 is available in your area (I thought I saw a post where it's available near you), I'd go with that year-round in your '88 300E. Otherwise, I'd go with the 5W-30 in the winter and switch to 15W-50 in the summer.
Ten years ago we had a doozy of a winter here and I ran 15W-50 in my car for the first 3 months of, what seemed like, a 9 month winter. I did notice a noticable reduction in cranking speed. So unless you have an oil consumption problem, I'd leave the 15W-50 oil for the warmer months.