Best Oil for 97 S500
 

I recently got a 97 S500 coupe with 50,000 miles on it, dealer serviced, maintained to a T....after spending about 5K on an a/c compressor, driver side window and suspension work, it's in really great shape....my question is this, My mechanic recommended I run regular 20w50 in it, and change every 3000 miles, he stated that the seals are somewhat thin on that engine and does not recommend synthetic.....I was wondering what everyone else runs or recommends in it....I'm thinking about just putting castrol GTX 20/50....any input?