Matt, normally the only disadvantage is that your oil may not have as much life in it. But D1300S is a Heavy Duty oil, & is designed to handle more abuse than a gasoline oil. I'd think it would go 6k miles in your engine on its own, without any M1 added. So, really, I guess I don't see any disadvantages to running this combination in your particular application.
Patman, I agree that there should be finer divisions between viscosity grades. Since, as you pointed out, we're the Anal Minority grouped together on BITOG, we can do whatever we want here & know what it means. In addition to SAE vis grades, we can incorporate mid-level SBF (Society of Bob Followers) vis grades which overlap the established divisions. This would give a better feel for where an oil starts out in life. I still feel we should be looking at thinning & thickening in some context other than vis grade "groupings". Since the grades aren't linear divisions, if we want to keep referring to SAE/SBF viscosities, then we really need a formula for converting cSt to SAE/SBF.