The reason that straight 30 was specified in some engines was to avoid the viscosity improvers that are required to make a 10W30 oil. Viscosity improvers are long chain polymers that react to temperature and allow the viscosity swing of a 10W30 oil. The viscosity improvers "thicken" the base 10 weight oil to make it like a 30 when hot. Magic. Viscosity improvers can also cause problems with deposit formation in the piston ring belt area causing ring sticking. So....some heavy duty engines specify non-viscosity improver oils....like the straight weight 30.
Ever notice that multi-vis oils were pretty much taboo for heavy duty diesel engines until the early-mid 90's?? That is because of the viscosity improvers. VI packages of the late 70's and thru the early/mid 80's were notoriously bad for ring belt deposits. Due to the high concentration of VI in the 10W40 oils those were especially bad.
Today this is all pretty much ancient history. The viscosity improvers in today's oils are all synthetic and there is little to no risk of ring belt deposits and ring sticking with multi-vis oils. Also notice the proliferation of 15W40 diesel oils that meet all diesel specs. The VI packages are excellent. No real need to avoid them these days.