Exactly.
Now, a couple other interesting facts: To make a 15W40 oil, they use a 15 weight oil, and then add compounds to it which "thicken up" when hot... effectively.
This means two things:
1. As the original oil is a 15 weight oil, the "film strength" is that of the thinner oil, which is lower than that of a thicker oil. (*NOTE)
2. The thickening compounds do "wear out", especially if the oil is sheared constantly - not so much of a problem in an IDI, but in a 7.3 PSD, where it's pumped at high pressures, it does end up thinner over time.
Now, do these really affect you? No, but it's interesting. 15W40 seems to be just fine for an IDI, even worked as hard as possible. Bearing wear due to use is not a big problem; you typically find bearing issues when someone, oh, lets the engine run dry of oil... or just plain doesn't change it at 5K(or less) and it gets all sludged up.
(*Note: Two interesting bits here - Because of this, a 5W40 has a lower film strength than a 15W40 oil does. However... Most engines have plenty of "reserve film strength", in that they aren't pushing the oil to it's limit. In something like a top fuel dragster, yeah, those differences probably matter. In an IDI? No; there's plenty of bearing surface area to handle the load).
