I agree with you Jim, multi-grade oils definitely have their place, particularly in places with cold winters to eliminate the need to change oil seasonally. Though personally, I would use a block or pan heater before I used a multi-grade oil.
What is this API rating you have that shows a multi-grade oil is "better" than a single weight? In years of research, I have never heard of such a thing.... I use Chevron DELO 400 S.A.E. 30 and it meets the EXACT SAME SPEC as Chevron DELO 400 15w-40. Same API seal on the bottle.
It's beyond coincidence that nearly every single person who has posted that their engine leaks oil or their road draft tube slobbers oil out at highway speeds, uses 15w-40 oil. That's my point.... It's not that 15w-40 is "bad" (though I don't use multi-grades for anything except my Honda Accord). It's that the Multifuel engines don't "like" it. Switch to S.A.E. 30, leaks stop.
Here are a few oil spec sheets to compare.... The first is for Chevron DELO S.A.E. 30 and the second is Chevron DELO 15w-40.
https://www.cbest.chevron.com/generated/MSDS/PDS7668881.PDF
Note the "bullet point" words they use in describing the single-weight products: "Long Oil Life", "Long Engine Life", "Superior Engine Cleanliness" (due to no contamination by multi-grade polymer breakdown residue), and so on....
https://www.cbest.chevron.com/generated/MSDS/PDS7668634.PDF
Now note the "bullet point" words they use to describe the multi-grade product: "Reduced Operating Costs", "Reduced Inventory Costs". This is because the product allows owners to compromise, and use the same oil, year round, for all their equipment regardless of individual equipments' requirements. Note that nothing is said of decreased wear, long life, etc. Why? BECAUSE COMPARED TO SINGLE WEIGHT PRODUCTS, MULTI-GRADE OILS ARE INFERIOR.
Also note that the single-weight products meet all CURRENT diesel engine manufacturer's specifications, like CAT's "ECF-3" spec, etc.
Regards,
Jon