Why do some engine manufacturers specify SAE 30 oils while others want 15W-40?
Almost all heavy duty diesel engine makers now favour SAE 15W-40 engine oils. The benefit is generally a lower oil consumption plus lower wear partly as the oil circulates faster after a cold start and partly as a multi-grade can also generate thicker oil films in bearings and between rings and liners than a straight 30 or even a straight 40. There is no engineering reason why a 15W-40 multi-grade using the current best technologies would not perform at least as well as an SAE 30 mono-grade in almost any engine. Monograde oils are, however, still preferred for some older 2-stroke diesel engines.