I recently purchased a '94 S500 (in March) with 76,000 miles on her (79,082 now) and was wondering about the type of oil for her. As far as I know, she has always used Castroil 20\50, and I was wondering if it would be advantagous to switch to a 'syn'oil? The engine runs great and have had NO problems, but I was told that 'syn' oil would be better for her. I was also told that she has too many miles on her to switch now. Opinions anyone? Thanks!!!