I've been using just the 5w20 syn. It stays clean and in good shape for well over 3 or 4k, but I never let it get that far lol.
What kind of mileage are you getting with the 0w40? I work far enough from home to worry about gas mileage now. I was actually thinking of switching over to the Mobil 1 0w20, as it states that it meets the same cold-start and hi-temp lubricity standards as the 5w20. Is the 40weight not a little thicker? I'm somewhat green in the areas of oil specifications and testing, I just have a pretty good handle on what's good and what's junk.
Thanks for the tip!