Actually there is evidence that increasing viscosity over 100K miles is not a bad thing. Some years back after my brother in law used some Walmart oil in his diesel. The engine made new noises so he changed it.

It could have been a mislabeled batch. Chrysler advocated a 10-30 oil in there new smaller truck diesel with bad results.

The side thrust loadings on the pistons are higher on a diesel. You want the oil cushioning effect.

We live in the Canadian Maritimes where really hot temperatures are not that common. All the old guys like myself believe nothing less than 15- 40 weight oil in old diesels.

Manufactures today want the lowest viscosity they think they can get away with. To enhance fuel millage.