I have a 1995 modified SL500. It a Renntech 6.0 engine conversion with other modifictions from brabus and lorinser. The dealer over here is a little too shady for me and they are telling me to go with regular petroleum-oil all the way or the engine will leak because it has to adjust to synthetic oil, but When I contacted Renntech, synthetic oil is highly recommended, they told me to use synthetic if I wanted the max performance from the modified engine. What's the grade of Synthetic oil recommended for the newer SL 500. 5W-30, 10w-30, 5W-15. or is it something else? I drive this car only in the summer. I garage it during the winter when I move to my second house in Florida and drive my other cars.