You absolutely can't make that assumption, I agree.
Mini-rant here:
This is a complex subject, lubrication. Any efforts to dumb it down destroys the necessary nuance, be it via results of a singular test like TEOST 33C or IIIH, via ingredient selection, like PAO or AN's or esters, or via basic approvals predicated on pretty mediocre performance requirements (API) or sometimes even high performance requirements (LL-01) that necessarily restrict formulation flexibility (Mobil 1 Full Synthetic 0W-40 regaining LL-01).
I've made it abundantly clear over the years that I don't think too highly of the basic API approvals. Given how mediocre some of the oils are that meet the performance requirements, I think that's a reasonable position. It's the same reason GM developed dexos. The Euro marques have always been ahead of the curve here with their own demanding performance tests layered on top of the basic ACEA protocols. Yes, you can buy an oil that's just A3/B4 for example, but if you own a Bimmer, Benz or VAG product, it's not going to be appropriate for any of them. I think this was well-evidenced in a past discussion that was had about the appropriateness of Euro approvals in non-Euro applications, where I made it a point to flesh out the fact that dexos includes several of the Euro OEM engine tests.
The API restricts phosphorous because it also only restricts Noack to 15%. Phosphorous is not restricted on heavier oils (xW-40 and above) because they are less likely to end up in the exhaust system. xW-16 and 0W-20 were both exempt from TEOST 33C under API SP (despite my opinions on TEOST) and xW-16 and below were exempt from Sequence VIE, while xW-20 had a higher limit. xW-16 was also exempt from Sequence VIII. This underscores that the API performance standard isn't a monolith, thinner oils, while having their AW chemistry limited, are also either exempt or provided with higher limits for several of the test sequences.
Historically, most of this hasn't applied to Euro oils, which were driven by the requirements of the OEM's on grades that were generally exempt from the API limits, but as they've embraced thinner oils, this has caused some issues. We've seen how this has played out for both OEM's and blenders, with many of them producing products that are "API SL" to side-step the phosphorous limit, or, like Mobil does, notes "meets API SP engine test requirements" while the product itself isn't API SP due to non-compliance.
All that said, the API does establish a minimum performance framework, and that's generally good for Joe Average, as it establishes a floor, even if that floor also inadvertently restricts formulation flexibility. When we look at products like SynLube, City Star or basically anything on the PQIA watch list, this importance is emphasized.
The API is a bit like the FDA. McDonalds passes the FDA requirements, so does Taco Bell and all manner of food that will shorten your lifespan. It's a minimum level of QA, not a Michelin star.
Boutiques that choose to operate outside the API framework have a whole host of different approaches they can take, depending on their intentions. Boutique, in my mind, does not imply high quality just like name brand doesn't imply mediocre. Mobil 1 isn't McDonalds. There are boutiques out there making up grades like their oils are bespoke artisan creations not beholden to any conventional standards, those are not products I'd be interested in or assume are actually high performance. It's like a guy jamming together speakers in his garage, there's no guarantee that these things are actually going to be good.
Some boutique blenders, like AMSOIL and HPL, work with the additive companies (Infineum, Lubrizol, Afton...etc) to purchase additive packages that they then blend with high quality base oils and VII's (from those same additive companies), then run their own testing to further improve the performance while top-treating. So here, you are starting with components that were approved and then further refining the product, improving its performance. This can also allow for the creation of unique blends, like the A3/B4, LL-01, A40, 229.5...etc full-SAPS additive package that HPL blended into a 0W-20 for me.
On the other hand, we've seen some boutiques recommend their products for conflicting specifications/approvals, which is a red flag. Synlube is "boutique", and hot garbage. I wouldn't trust a thing Lucas sells due to their perpetual "oil stabilizer" grift. Anyone remember the saga of
BND "Quantum Blue"?
