I just got my first b-service on my 99-clk 430 this past week...i saw on the sheet of paper they gave me that they put in castrol oil...i was wondering if that is synthetic oil they are putting in or is it just a mix...since i don't drive my car very much (21,000 in three years) and i follow the FSS counter i usually get my oil changed once a year...i thought maybe i should be using synthetic oil instead...maybe something like mobil 1....thanks in advance...