and just what fricken surfaces in a diesel engine are "closer together" enough
so that 30W oil will flow and lube but 40W won't??????

If that were the case, those "closer together" surfaces would not get lubed at
all until the engine warmed up enough so that the cold 30W would flow.

Even the hydralic valve lifters on a gasoline engine (with some very tight
tolerances, down to a few thousands of an inch) would not operate (what a hell
of a racket you would hear) on 40W or not-yet-warmed-up-to-operating-temp 30W.

Oil too thick = may not lubricate completely between surfaces that are
closer
together, where the lighter oil is designed to flow.

this is not true in an engine. the difference is important during cold
engine
startup.

Are you daft? (retorical question)

BB