I do some work at a local truck manufacturing facility occasionally. The engines show up on the loading dock dry and are filled once the truck nears the end of the assembly line. They are currently all getting Delo 5w40 synthetic.
I honestly don't see the issue switching to synthetic or a blend right away. And what the marketing on the label mentions VS what is really in the bottle is 2 different things. A lot of synthetic oils marketed in the US are what the oil industry still considers a traditional oil and are group 3 severely hydro-cracked base oils. Depending on the build and machining there is good cause for a brakein oil, or at least some breakin additives, but I think this entire no synthetic in a fresh engine is the same hold-over and partially mis-information as the synthetic oil causes oil leaks. The oil leak was already there, the synthetic was just able to flow thru places the crusty dino oil wouldn't. Additionally it depends on what kind of "synthetic" you had a leak with as they don't all have the same chemical properties. For instance, the ester base oils used in engine oil have a natural tendency to swell the elastomers typically used in seals and would help seal leaks not cause them. Conversely, POA base oils can tend to shrink those same materials and you have to add swell agents to the additive package with those (also frequently added in the ad packs of high mileage oils).
