Here is a useful link: https://www.reference.com/vehicles/understand-motor-oil-weight-9a73155f28c8df87?qo=cdpArticles
And for the point Panic makes: http://www.valvoline.com/auto-resou...hincognito.com&an=msn_s&am=exact
As often happens, even the "experts" can be , not-helpful. Little "w" is winter rating(cold?) And "big W' is "weight"? Density, viscosity and molecule size...are we having fun yet?

Steve H