Whats the difference between 5w30 and 10w30 which ones better?

5W30 means that it is supose to have a viscosity of 5 when cold and 30 when warm. The W stands for winter. Of course 10W30 means a viscosity of 10 cold and 30 warm. If the temp in your area gets realy cold then 5W30 would beat 10W30. The auto manufacturers started using 5W30 in new cars to help meet emissions and fuel economy standards.

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'69 RS/SS396 pro street
427/4spd/9"
Byars Performance
High Performance Drive Train Parts And Service
www.lubedealer.com/biggearhead (http://www.lubedealer.com/biggearhead)

When I bought my engine from American Speed they suggested straight 30 weight oil unless I was actually going to drive the car in cold weather (not planned).

Mark M.