If that's your experience then by all means you should act on it, I can only relate my own... and in that I've noticed advertisers do tend to exaggerate a bit, not all manuals include every variable, chemical technology has advanced a touch in the past forty years and there's no source better than someone whom actually uses the product in question, yet has no profit motive.
As an example, both Yamaha and Mercury (marine) have warranty disclaimers stating that you must use a specific brand
of oil, not just type. I find it interesting that both companies also have branded oils, Yamalube and Quicksilver - coincidentally, these are the oils recommended. Now, they state this is because they've tested the oils to insure it meets or exceeds specifications and I'm sure this is true, you've a point about warranty repairs and loss of profit, there's even their reputation to consider - they wouldn't want to be known for untrustworthy engines for the sake of selling a substandard oil.
So - they've told the truth... but not all of it. Did they even test oil alternatives? Why would they go through the bother if they're only going to end up recommending a competitor? These are highly reputable companies - but they're not charitable.
Even personal experience is subject to this, for instance many of my friends as well as myself are NGK advocates - we swear up and down there's nothing better in a two stroke than a NGK plug simply because it isn't as prone to fouling. Thinking about it one day I realized I'd been using NGK for over over twenty five years, that due to my product loyalty - I didn't actually have the cross-experience to back this observation up, I simply didn't know if it was
any better than anything else - only that it was what I
considered good enough. Curious, I wandered around and asked my other motorhead friends, marine techs, snowmobile & dirtbike fanatics. They all said the same thing, that NGK ruled and they'd use nothing else - when asked if they had
used anything else to compare their favored product with, they pretty much all answered with a surprised no... those few that said yes had only done so because that's what the engine came with - not exactly fair as they're usually used or a cheap "stock" item.
So how do you know who to listen to? Ya can't really - no any one source has the whole tale to tell. Best you can do is to listen to all
sides w/a touch of educated skepticism, knowing all statements have a bias - even if it's just opinion.
Still, that's what I love about this forum - there's a bunch of experience shared without a commercial interest and it's fairly easy to determine general knowledge vs product loyalty. Despite my Amsoil preference - due to the statements made by any number of folks that use Opti2 on a regular basis, have tried other oils yet still recommend it - but most importantly have no profit motive... heck, I'd love to run Opti & the only reason I haven't is it's not commonly available around here.
Does this mean you should let people convince you to go against your experience? Of course not if you're not comfortable with it, but does it mean they're wrong because they don't agree with you?
No - there's quite often more than one "right" way to do things after all