This is a very interesting article that blows holes in many myths and misunderstandings about motor
oil - particularly with regard to viscosity and the debate between dino and
synthetic lubricants. Well worth reading:
Motor Oil 101 - Bob is the Oil Guy - Bob is the Oil Guy
After reading this I'm thinking switching from 20W50 dino to 5W30 or 5W40
synthetic, depending upon running
oil pressure in a hot engine. Hint, hint...