My 95 S320 is designed to idle fast until the engine warms up a little. It is in the manual. On the subject of oil weight you'll get a lot of opinions, but very few will suggest 20W-50 if the engine is in good shape. Most will suggest 5W-30 to 0W-40. Many will suggest Mobil1 synthetic. That's what I use.
1984 300D Euro