I've been running the Chevron 15w40 for a little while now and am quite happy with it.

I will be switching my wife's Subaru over to Rotella 15w40 synthetic this weekend, since I've read that it helps with the clattering some (all?) Subarus have.

The owner's manual specification tells you two things:
* what will void your warranty (if they even bother to check which is unlikely)
* what the EPA recommendation of the day is

The only possible bearing on reality it has is that an engine designer might build in tolerances according to the expected oil viscosity. Then again, they also forgot to put enough oil holes in the pistons. They're not perfect either.

So, use what works. Many Isuzu owners (planetisuzu) have reported good results with 15w40, especially Shell/Rotella synthetic resolving the oil consumption problems. I'm always inclined to go with experience over theory/superstition any day. Some people here have good experience with the Mobil-1 SUV formula 10w30, so that might be the way to go if you'd rather stay within the specs of your owner's manual. Change to the Rotella at the next change if it still burns oil.