If you want to do a little research, with specific specifications on cold and hot viscosities of dino and synthetic at various grades, I highly recommend studying

http://www.mobil.com/USA-English/Lubes/Product_Information/Engine_Oils_Automotive_Collection.asp

Lots of great information... For example, you'll learn that a dino 5W-30 offers exactly the same viscosity throughout an engine's normal warm operating temperature range as a synthetic 10W-30. Not saying the same protection of course.