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The chances of them destroying their motor with a 5w40 are pretty slim, but put 5w20 in an LS1 and run 300F oil temps, and you can kiss that motor goodbye pretty quick.
Come on Patman, who said anyone should run 5w20 in an LS1 and run it at 300F oil temps. In the same light, your GC at 300F is the same viscosity as a typical 5w20 is at 266F. Will the engine running 5w20 is at 266F self destruct significantly quicker than GC at 300F?