Oil

I had a mild 383 a long time ago in a 4-speed A-body, 3.23 gears... oil pressure was about 10 psi higher at both idle and operating RPM with 20W-50 vs. 10W-40. But it made an 0.3 mpg difference to the fuel consumption (averaging around 12.4 vs. 12.7 mixed city/highway). So the thicker oil was wasting some horsepower...