I'd say taking an engine that is knocking and smoking for a 20 minute drive had a lot more to do with it than the fuel used. Sounds to me like this engine was on it's last leg anyway. I'm betting you didn't baby it on the 20 minute drive either. Sounds to me like you wanted this engine ruined.

"It's just a phase. He'll grow out of it." Mama, 1964

1956 Chevy 1/2-ton 3100
1953 Chevy 6100 "The Yard dog"
1954 GMC Suburban Now with a new proud owner.