Looking back through my records, I tend to find the following (oil change interval, number of quarts burnt during the interval) 20,2 30,3 40,5 50,7 Is it really possible that older oil gets past the piston rings more easily? I use 50/50 W80 and Shell 15W50, plus Camguard which has produced a big drop in the “hard” metals wear rate (according to oil analysis).