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.
'69 RS/SS396 pro street
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