Originally Posted by Godzukidan
As far as the low number. If your car calls for 5w20 it will not hurt to run 5w30. But I have heard do not run 10w20 or 10w30. I guess the low number is how thin the oil is at normal temp. And the high number how thick the oil is at start up. I to would like to know more info on the the numbers. Maybe someone with more info could clear this up.
I do know when I used the Mobil 1, my car at idol was real smooth. I would use it all the time if it did not cost so darn much. I change my oil every 3000 miles and it can add up.
That's not quite correct, the lower number is the viscosity under winter or cold (below 32F) conditions and the higher number is the viscosity at higher temperatures (above 210F).
A 5W20 and a 10W20 would behave exactly the same way at 210F at least theoretically, however on a cold engine the SAE 10 weight oil would be thicker than the SAE 5 weight oil which is bad for engine wear at start up.
However with viscosity improvers comes the risk of shearing and breaking down the oil to becoming thinner at higher temperatures.
You want the oil to be thicker at higher temperatures to prevent it from flowing away from the metal surfaces but not too thick as to cause flow issues, or damage from increased pressure.