I have been running Castrol full synthetic 10W-40 year round in this Mercedes since I purchased it. The previous original owner used dyno in it. I like the way the synthetic has done. It is very quiet, and no big contamination or wear indications. I was just thinking if the Royal Purple was much more enhanced, that I would bump the bar. Also, if it gave better protection on some of the rubber and plastic internals it might be worth the investment. Thanks, for the information on it!