I am picking up a 92 5OOE on the used market. The car has approx 100K miles on it. Its led a pampered life thus far and I want to make sure it lasts as long as possible. What are the virtues of synthetic vs conventional oils in applications like this? What about oil weights? How does 15w40 fare against 20w50? What about those fully synthetic oils like Royal Purple? I've heard about synthetics being "damaging" to older cars because of their greater detergent qualities dissolving the "false seals" in the engine. Is there any truth to that and how could one tell if they had an engine which would either benefit or be harmed by synthetic oil. Thanks....