To make a multi-grade oil, you take a single grade oil and add special polymer additives called viscosity index improvers, or VIIs.
For example: a 10W30 starts off with a base SAE10 grade oil. The VIIs make it perform like a SAE30 grade oil at temperature.
https://media.noria.com/sites/archiv...3_BP_Fig_1.gif
Problem is when those VIIs break down, your multi-grade oil behaves as a single grade oil. In the case of a 5W oil it will behave as SAE5 at temperature.
VIIs explained:
To summarize: 0W and 5W manufacturer recommended oils are to meet CAFE standards and have nothing to do with "tighter engine" tolerances, etc.