Assuming you are referring to SAE Viscosity Grades for Engine Oils (SAE J300) there is no such thing as 0 or 5 grades. What there is is 0W and 5W. W grades are defined by their low temperature performance (as gauged in the Cold Crank Simulator and Mini Rotary Viscometer tests). Both grades require a minimum Kinematic viscosity at 100 C of 3.8 cSt, but have different low temperature requirements.
Non Winter grades are characterized by their Kv100 and High Temperature High Shear rate viscosities and don't require low T testing. Multigrades require both.
I haven't ever heard of commercial W-grade monograde products, but for example if one had a lubricant with Kv100 of 3.9 then it could be a 0W or a 5W monograde depending on its low T performance. It would not be a 0W-20 or 5W-20 because those require both the applicable low T performance AND Kv100 of 5.6-9.3 cSt (along with HTHS at least 2.6 cP).
Understanding viscosity behaviors and specifications can be confusing, and that's a big reason why the SAE grading system has been so widely adopted.
