Which one is better??

 

All depends on your climate. Hotter temp areas tend to run thicker oil, where as colder tend to run thinner. Heres a case in point:

In my Nissan 240sx, I have a turbo motor swapped into it. While living in Texas, during the winter it got to about 60 avg, and during that time I would run Mobile 1 10-30. During the summer, it could reach temps up to the high 90's low 100's on very hot days. During those months I'd run Royal Purple 20-50. Here in Germany it doesnt get nearly that hot, and in my Nissan 200sx with turbo motor, I run 15-50.

 

Depends on the specific oils involved M1 the ever popular and POOR choice the 5w-30 is actually thicker at operating temps than the 10w-30 and both are very thin for 30wt. oils, too thin for this engine. That statement is based on actual wear metal content in used oil so in M1 you are much better off with 0w-40 or 5w-40, one of the best oils going for the LT1 and LS1 is a specific 0w-30 which is nearly as thick as a 40wt. and some 5w-30s are perfect for this engine. Go here http://theoildrop.server101.com/ubb/...ubb=forum;f=26 and do some reading for real oil info there are only a few of us on this forum who will make statments based on actual fact as opposed to "works for me" logic with no data to back it up.