The W grade indicates that it has met two low temperature viscosity requirements and one high temperature (100 C/212 F) viscosity requirement. SAE J300 sets the standard.
2015 Table here:
http://www.widman.biz/English/Tables/J300.htmlAn article on viscosity standards is here:
https://bobistheoilguy.com/putting-the-simple-back-into-viscosity/"o A 10W-30 Group I mineral oil is basically a SAE 10 that has polymers that expand and cause resistance when heated to flow more slowly, acting as a SAE 30 in hot areas of the engine.
o A 10W-30 Group II oil is similar to the Group I oil except that it is much stronger molecularly and therefore uses fewer polymers.
o A 10W-30 synthetic oil [Group IV or V]is basically a SAE 30 oil that has been created structurally to act as a SAE 10 when it is cold. It does not need polymers."Richard Widman Selection of the Right Motor Oil for the Corvair and other Engines 24/05/2009 Page 5
