Changed my 2006 Civic over to Mobil1 5W-30 (at approx 1500 miles). Had a 5qt jug of the SL variety around and wanted to test the '30 weight' concept in my civic before I change over to Amsoil 0W-30 in the fall. Interestingly, the owners' manual didn't say anything foreboding about leaving the factory fill wonder oil in there until the first OCI...wouldn't have stopped me anyway. I'm a little jaded on the whole "manufacturer knows best blah blah" when they call for different oils depending on which country the cars are shipped to.
I've also done two drain/fills with Amsoil ATF, so the trans is about 65% syn.
Here are my impressions:
1) Significantly quieter and smoother. With the 5W-20 it sounded like a can full of rocks sometimes. Sounds and feels more pleasant at all speeds / loads / throttle settings.
2) Significant rattle from engine compartment at 1400 rpm is for the most part GONE. It's either being muffled or the engine vibration freq was changed by the slightly thicker oil and it doesn't happen as much.
3) Engine does not 'spin up' quite as fast rpm-wise. Before, the engine would start screaming into the higher rpms with a fairly light touch on the pedal...often more than desired. It seems easier to 'drive smoothly' with the 5W-30 because the rpms don't go through the roof as easily.
Overall - I think I prefer Hondas better with 5W-30 vice 0/5W-20, based on this experience and my experience with a 2003 Odyssey that I ran 5W-30 in. Since wear metals seem to be about the same, I'm OK with sacrificing the minute difference in fuel mileage to get the smoother engine sound & response. I recall my '98 Civic (with 1.7L VTEC) was very smooth with 5W-30 too.
Transmission is smooth as can be with the Amsoil ATF.
On a side note, had to bust out the Channel Locks to get the #@$* filter off! Fortunately it's an easy reach on this new engine.
YMMV, Matt
PS- Happy New Year! I resolve not to buy any more new cars in 2006