Ml320 Oil Change Interval And Oil Type

[DEPGOODY]

Just discovered this board recently. It is great! I leased a 98 ML320 when the flexible service interval called for infrequent oil changes. Being an old car buff, this seemed too long an interval. I changed it instead at 5500 mi intervals since new . Vehicle now has 41000 and uses no oil. Since almost new has a slight valve train noise. Dealer says it is nothing . I have a good option to buy shortly. I have always used regular oil. Would it be wise to switch to synthetic oil if I decide to keep this vehicle? If I do purchase I would get an extended warranty. Which one is best? Thanks for any help Danny