I gotta back Moe on this one. On high mileage engines, switching to synthetic could be a bad thing -- you will probably spring an oil leak somewhere where you didn't have it before. The M103's are good for 300k+ miles easily, no matter what type of oil you run in it -- as long as you change it regularly.
How many miles on the engine?
'91 300E, 207k miles (20w 50 Pennzoil)