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It's M1. You're fine. There won't be a discernable difference in FE between the two. But 21k miles on oil? I salute you, brave one! I don't go longer than about 9k miles on full synthetic in my car since it's getting very nasty after ~8500 miles.
If you haven't changed them yet, I'd look into changing your lower intake manifold gaskets. Those can leak coolant into your cylinders when they go bad.
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2012 Chevrolet Cruze Eco 6MT
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