SLF3,
Unofficial SOP’s have more to do with technique, that is, the different ways an aircraft can be flown to achieve the same end. There are at least three ways to perform a crosswind landing, for example, but only two of them I know that work well for the tires. Some companies mandate one or the other in their ops manuals, I prefer to leave it to the discretion of the operating pilot.
The agenda some posters here seem to have, that being the deliberate disregard of manufacturer directives (they only built the thing, they don’t fly it), is something you rightly object to. In the case of your complex power plant, I don’t really know how many of your procedures are open to interpretation, but I would guess not many. In the operation of large aircraft, we follow the strictest of manufacturer-generated guidelines, all of our emergency and abnormal procedures are standardized, sequentially itemized in our checklists, and cross-checked by both pilots (ideally) in the performance thereof. It is not the only way to fly, but it is far and away the safest and most lawyer-free.
Anyone on these pages who tells you different is talking sh@t.