Engine oil weight for a particular engine is determined by duty cycle as much as anything. If you were racing the vehicle or going to be running it at high speeds for long periods of time, I'd say 15W40 would be OK, as once the engine is warm, it's going to be thin enough to do what it needs to do. If it's not going to be run hard, I'd steer clear of 15W40 for an engine marked for 10W30. The key is operating oil pressure, which is inversely linked to oil temperature. If your engine oil pressure at operating temps is where it should be, you're okay. Read AE Haas' "dissertation" on this before you do it though. Motor Oil 101
If it's not going to be run hard, I'd steer clear of 15W40 for an engine marked for 10W30.
Couple points...
Most engines are spec'd for a range of weights... and almost every owner's manual indicates anything from 5w30 to 20w50 in a gas engine (all of mine did).
The 5w30 (and lighter weights) and their "recommended use" are often regarded as being more for meeting CAFE numbers (read: higher fuel mileages) than engine protection. This isn't saying a 5w20 or 5w30 won't possibly protect as well, but they aren't spec'd by the manufacturer for longevity, but economy.
I run 5w40 Rotella in my 2009 Forester XT (turbo) with good results... I run 5w40 Schaeffers in my 97 Outback with real good results. Most German/European cars are spec'd for 5w40...
My new 09 Challenger R/T w/5. 7L Hemi, 6 speed manual (therefore no MDS) specs nothing but 5W20. I ran 0W20 Mobil 1 this winter, and the guys at Wallyworld were warning me I could have problems (a few used to work for local Mopar dealers). Appparently, there's a grain of truth, as far as the MDS goes, but some have said that there's a TSB out for overseas vehicles to allow 5W30, even though the 5W20 was spec'd for the MDS (valve system runs off oil pressure. ) and CAFE requirements.
I told him it was a 20 weight oil (syns are based on the viscosity of the oil at op temp, versus "dino" oil, which is based on the viscosity at 100 deg F and they add modifiers for cold start) so it should be fine, besides the package says it meets the DC spec and actually exceeds. Never got a MIL to light up. Went to 5W20 on the 12K oil change since it's summer.
I wouldn't be afraid to run 0/5W30 in it, since there's no MDS, and the pre-MDS hemis ran 5W30 from what I understand.
As for the link I put in regarding oil viscosity and grade, sure, that's from a guy whose experience is in high performance racing engines, etc, but at some basic level it applies to any ICE engine.