Thanks for the info. I had always used 20W-50 dino oil year round here in NE Florida (15W-40 in winter in diesels) until being forced to use synthetic for the newer FSS cars. I will switch to 15W-50 Mobil 1 now for my self-serviced vehicles and for the extra changes I do on the in-warranty SLK (I can't see changing oil only every 11K-12K miles whether it is synthetic or not).