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CCS and MRV measure different oil properties. The CCS test measures the ability of the oil to be picked up and pumped by the oil pump under shear stress. MRV measures the ability of the oil to flow under its own weight back into the hole sucked out by the oil pump.
Think of Jell-O. Jell-O can be easily sucked up and pumped by a mechanical pump, in fact you can suck it up through a straw. But once pumped the remaining Jell-O will not flow back to fill the hole, and the pump (or straw) will now suck air. Jell-O, like many oils, has a weak crystalline structure that can be easily overcome by applying shear, but without such external mechanical force it cannot flow.
Both tests are influenced by viscosity and wax crystal growth/pattern, but they differ in shear rate, i.e. higher shear pumping versus lower shear flowing. The actual conditions under which the tests are run were developed to correlate with known oil performance observed in the field and has proved very useful over the years.
I would consider CCS to be less critical than MRV since modern oil pumps are quite capable and efficient. The factors that influence MRV are more elusive and sometimes unexpected issues arise during formulation.