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.
For those about to ROCK............. We salute you..! (AC/DC)
L.O.T.H # 67