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Generally speaking the shorter the swing between winter rating and weight the more stable the oils viscosity will be. However, the fact that 5w20 is more stable then 5w30 doesn't mean it protects better at anytime throughout an interval. Because for the most part the 5w30 will still likely be higher in viscosity and film strength even at the end of that interval. Oils mix no problem, I would mix that 5w20 instead of using it personally, and then just buy 5w30 from now on.