Here's my latest German Castrol (0w-30) story. (It gets good at the end.) I bought up about 20 quarts on clearance at Advance about 4 or 5 years ago. Its the 2nd gen gold colored, not the green colored. Its been sitting around not doing much. My wife's car has about 180k miles on it. It developed some sort of a tapping noise at idle at 80k or so. It was getting annoyingly (worryingly) loud of late. It calls for 5w-20. The car has variable valve timing on intake and exhaust (DOHC.) I reasoned that with high miles and a steady diet of Quaker 5w-20 ultimate (don't ask) there might be something worn that could use the cushion of a quite thicker oil (at operating temp.) And I thought it couldn't hurt running the 0w-30 GC and the worst case would be increased fuel consumption. So, in went the old but gold German Castrol. Been driving it around for a few weeks now. The tapping has decreased to the point that I don't even notice it anymore, although it is still there if I listen for it. It USED to WORRY me.
SO, I decided to do a steady state fuel economy test. Windows up, no AC, relatively flat ground over 25 miles or so @55mph constant. Its an economy car and the owner's manual calls for "87 octane minimum." Using 87 octane from new, the car always got about 36-37 MPG. I once got a freak reading of 41 MPG, but that was after switching to premium fuel @ about 120k because I was getting audible ping on 87octane. Further tests with premium were back at 36 or so. ...With the new 50-cents-a-gallon extra for premium that all gas stations went to this year in my area, I decided to go back to 87 octane in the middle of the summer. The ping really didn't come back for some reason but I know it was hurting my economy by watching the timing on a monitor plugged into the OBDII port. ...But NOT $5 per fill up "hurting." The one MPG test I did with the German Castrol a few days ago was with 87 octane and the ambient temps have cooled off lately and helped with the ignition timing, but the timing still might not be ideal. NEVERTHELESS, with the German Castrol, the car got exactly 50 mpg! 9 mpg better than I've ever seen. And on 87 octane. The only thing I can figure is the thicker oil is giving more authority to the variable cam timing. OR, is the real, old German Castrol really MAGICAL?