OP, I've been asking myself the same questions.
Since you live in North America, you have access to some good monograde oils that also have up to date API SP rating. It's very unlikely that you will get more advanced specs on a monograde, such as Dexos, MB 229.5 or even ILSAC GF-6, as these specs require a fuel economy test or maybe cold starting test that is a monograde's weak spot. This cold start requirement is often indirect by only allowing certain viscosity grades (always multigrades) to hold the approval, e.g. Dexos1 allows 5W30 but not 10W30 no matter how good the 10W30 is.
But apart from starting in very cold climates (below freezing) and the associated warm up fuel economy hits, I think monogrades have a lot to offer. Such as no viscosity shear and very low Noack volatility.
As for synthetics, the more common cheaper synthetics multigrades are good, but commonly Group-III based. But Group-III base stock, even GTL, only comes in 2 cSt or 4 cSt or 6 cSt flavours. So to make a Group-III 5W30 with a KV100 of 10 cSt requires you to thicken it up with polymer viscosity index improvers (VII) and therefore must be a multigrade.
So monogrades only come as three types:
* Group-I, old school, bad oxidation stability and very bad cold characteristics. You don't want this.
* Group-II, viscosity index (VI) over 100, a modern mineral oil with better oxidation stability and better ability in the cold. A good choice. Often under tested (API SG because it's easy, and then sold as small engine oil). I think most of these could easily be SN, but there is no profit in doing more expensive tests on what is sold as a mower oil. Mag-1 make good SP rated SAE 30 and SAE 40 monogrades.
* Group-IV (PAO), fantastic but rare and expensive. A PAO SAE 30 could probably pass as a 10W30 monograde and I believe Amsoil has one.
More monograde talk here
Posted courtesy of @53' Stude
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