If you call 1-800-FOR MERC they will mail you a dandy free brochure on recommended fluids and other service products.
Having said that, one thing for sure: no 5W30 or 10W30 oils are approved by MB.
Personally I use Mobil 1 15W50 year round. Despite the high-sounding lower number, it remains fluid down to very low temps. No trouble starting this winter despite below zero F temps.