The detergents in synthetic oil aren't going to work overnight. It'll take awhile for them to clean stuff out, if anything is clogged. I would try using a diesel oil, like Delo (conventional) or Delvac/Mobil 1 Truck(synthetic, they're the same stuff), and change it every thousand miles or two and see if that helps.

Also try using a lighter weight oil. Mobil 1 0W40 flows pretty well.