My concern is that back in the early 2000s, without any stated requirement for ACEA, a 10W30 strictly in accordance to API only (in this case API SJ) back then might have had HTHS of lower than 2.6 mPas, in this case would it matter or not whether we chose an ACEA A3 or A5 oil anyway? (For a 30wt oil)As Kschachn alludes to above, all ACEA oils are required to pass a shear stability stay-in-grade test, this test is not part of API and one of the reasons I like Euro oils.
My current ride, Nissan X-Trail (aka Rogue) is known to shear oil, so I’m running Castrol Edge 5W30 A3/B4 for just that reason.
However I do agree, if you are comparing two similarly formulated API & ILSAC oils from the same company, you would naturally assume the 10W30 is more shear stable.
I found this API SP licensing document stating some of the requirements, interestingly enough, includes those obsolete/superseded ones from yesteryears.