10-30 was factory fill the original GTO's. I remember the 10-40 issues 40+ years ago. Forgot all about that! I've run 15/40 Rotella in all my flat tappet cars for the past 22 years with zero issues. I am in a very hot climate. I change my oil every 3000-4000 miles or 8 years, whichever comes first.
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Jeff 
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