I'll be the first to call bunk on the synthetic mythologies ...
Plenty of companies using it (in gassers) right from the factory. Are diesels different than gassers? In cycle operation, yes. But in machining technology and materials used, not really.
What's up with the 5 minute idle comment? Are you referring to the DPF cycle? Sure it burns more fuel during regen, but there are times when people leave the vehicle running for convenience; to keep the vehicle cool or warm. Long idle periods used to contribute to "wet-stacking" in older IDI diesels, but today's common rail, electronically controlled, injection systems don't have near as large an issue. It's there, but nowhere as bad as it used to be. And synthetics have nothing to do with this one way or another.
Further, I don't know of any "dealer" that order the trucks with "synthetic" for the Allison 1000 series in the 2500/3500 GM vehicles. There is no transmission fluid RPO; these come straight from the Baltimore facility with DEX VI in them now, which is stardard GM factory fill in all their applications for cars and light trucks. The Allison 1000 built in Indy is supplied for the TopKick/Kodiak trucks, and used TES-295 from the factory. But again, there is no RPO. You get what you get; there is no "optional" tranny fluid.
And where's the logic in suggesting not to use synthetic in the engine right away, but proporting it for immediate use in the tranny? Do you think trannies don't go through a break in cycle?
The rear differentials come with synthetic GL-5 in them from GM (commonly called "grape juice"). Would you suggest removing it for "break in", and then changing back to synthetic again???????????????
Synthetics extend drain intervals. Synthetics survive extreme temps better (and I mean E-X-T-R-E-M-E). Synthetics don't grossly affect break-in.
That's my take on it.