Well, a small vac hose had popped off, causing eventually a CEL (lean code). After reconnecting that hose, gas mileage went back up to an average of 24 mpg, which is about as good as it ever gets with 50% city and 50% highway driving, leadfoot considered. Power is restored, too. This confirms my previous belief that in this enegine I can't tell a performance or gas mileage difference between oils. The only perceived differences pertain to noise, smoothness, oil temperatures, and oil consumption. And DELO 400 makes my engine run as smooth as any other oil I've tried.