Originally Posted by tp72chev
You right that this change was brought on by government regulation but in order to make it work engineers had to set bearing clearances closer for a thinner weight oil to limit oil seer. If anyone on here was an engine builder instead of an opinionest they would know that. 15w40 will operate the same as a 5w30 (which is what your vehicle calls for) ONCE your engine reaches operating temperature but on cold start-up depending on your climate you live in is when it is too thick and CAN cause premature bearing wear over time. I think people have over looked the fact that you said 2005 not 1998.
I'm not sure what credentials you need to be called an engine builder, but by any standards at all, I am that. I've also been privileged to spend time with some of the best to learn and debate. I've built more than my share of many types. I've also participated in R&D work with several oil companies and independent labs. There are no tolerances in my 06 4.0 which, when subjected to the pressures and conditions we can see, would cause the damage you predict. The only time ,again, that we've seen this type of damage has been with a combination of much tighter tolerances and much higher oil pressures. Further, your claim that the two oil weights act the same is a bit off. A 30 weight oil is rated between 9.3 and 12.4 centistokes and a 40w between 12.4 and 16.8 (at 100 degrees C). So, by definition, the 40w is somewhat thicker. Exactly how much? Depends on the two oils. A 30w that comes in on the top of the scale and a 40w that comes in on the bottom will be extremely close; almost identical.
Run 15w-40 if you choose and don't sweat it.