That may not be true. For example, as I posted previously, Mobil shows using Group II+ to blend an HDEO 10w-30 (similar spread to 15w-40), additives are 20%
View attachment 78931
Here's a 0w-40, 5w-40 and 10w-40, same 20% additive content in the 5w-40 as the 15w-40:
View attachment 78932
Heavier one in this table, 20.7%, 8.7% being VII:
View attachment 78934
Compare to a Group III 5w-40 where VII load is 11.8% for the "balanced" formula:
View attachment 78937
Another HDEO, this 10w-40 has 9.8% VII using Group II+, more than our 5w-40 Synthetic above:
View attachment 78939
I'll see if I can dig up one that specifically covers a 15w-40.