90% of all wear is at start up. The oil is cold (ie not at its rated viscosity) to thick to make a good film. using a 15W-40 means that your start oil is much (I hate to use this word) thinner (really, I mean LESS viscous) and creates a proper film much more quickly. The 15 weigh it to 'thin' to run with on a hot day, so the 40 weight protect you much higher temp wise.
I can post you links all day long if you promise to read them.. otherwise I'm not wasting my time.
60+ year old manuals have no place except for the unique systems used in early motors. Most factory manuals from the 40's say use 20 weight oil (mineral oil) with no detergents (it wasn't invented yet!) or additives. So no, the manual is not always right. Should I still use leather seals? They work, but last less than a third as long as neoprene.
If you want to use 30 weight.. it's not 'wrong'... but there is a better option... because you want to keep that babbit in the bearings as long as possible and avoid an expensive re-babbiting.
