I have lots of 0W-40 motor oil that I used for my Audi. It is high performance Lubro Moly fully synthetic. Is there any reason why I should use only 5W-20 in my HAH?

2005 Accord Hybrid
2005 Durango HEMI (wife's wheels)
1998 AUDI A8 4.2Q

I believe the 5W20 is used to maximize fuel efficiency and, since it flows easier, the oil pump and oil passageways may be designed for it. Personally, I wouldn't chance 5W40 if the engine manufacturer specifes 5W20. And since Honda is the largest manufacturer of engines in the world, I think they should know.

Side note here: The factory 5W20 oil that I drained from my HAH at 4500 miles had the consistency of water. Thin...very thin. I could dip my finger in and it would drip, drip, drip from my finger tip just like H2O. Very slippery, but very thin.

The 0 part means its got the cold-weather viscosity of any 0 weight oil, and will pour even thinner than 5W20 when cold. That's good.

The not so good is that the 40 part means it will pour like any 40 weight oil at operating temps, which is most of the time your car is running. The passages and tolerances in the engine may not allow full oil flow if the oil is thicker.

So 0W40 will let your car start and run a little more efficiently while cold, but not as efficiently while at operating temp.

While you will see somewhat lower mpg and you are making the oil pump work a little harder, it shouldn't hurt the engine in the short-mid term. I wouldn't use it all the time, though.

Personally, I'd see if you could exchange the 0W40 for 5W20 (I use 1/2 dino, 1/2 synthetic). Or wait until 0W20 is easily available again- Pennzoil is supposed to release it in the spring. You can still get Mobil1 0W20 form NAPA's inventory.

gonavy nailed it.

Probably mostly talking to myself here ... I came for a bit of reassurance that what I did yesterday wasn't TOO stupid!

My J-Lube does their best to frustrate me. After telling me that I got their $79.99 synthetic oil change for my HCH '03 last time, and that I probablly wanted the same over-priced oil change again this time ... I agreed and proceeded to wait in line for about 45 minutes. Get to the front of the line and ... Surpise! ... they don't actually have any synth oil today.

Fuming, I head over to WMart. I remember it was 0W-something. 0W40 was the only thing on the shelf (Mobil 1). So, I buy 4 qts of Mobil 1 0W40 and a Fram oil filter, take the stuff home and do the deed.

THEN I pull out the manual. Ooops, not 0W40, should have been 0W20! Panic sets in, I'm going to blow up my precious HCH.

But then I check a few forums and it seems like the down/dirty is that I will get reduced mpg at normal operating temperatures (the 40 part). I guess I can live with that if it's not doing any permanent harm. I'll check NAPA (thanks, Forum) to see if I can find any 0W20 and change after 5,000 miles.

If anybody's out there, would they do anything differently? Thanks.

you're fine.
If you want, go and change it to 0w20 the 1st weekend you get a chance. No rush. You may not even see a MPG drop- let us know!