Which engine oil ???

[shadrach74]

What flavor said is absolutely true. Also I would add that the XXW number is a measure of of how the oil flows (or doesn't flow) at a 100df. The -XX number is a measure of how oil flows at operating temp 212df. So 10w-40 and 0w-40 perform (read flow) quite differently at 100df, but exactly the same at 212df. My primary concern is start up lubrication. Which almost no oil does adequately.

While I use Blackstone for analysis on several vehicles and enjoy their plain speaking way of explaining each annualized sample, the are still measuring trends.

There is no logical reason I can think of to run 15W multigrade in a water cooled engine with modern tolerances unless those tolerances have widened to such a degree that it is required to maintain pressure. Even if i lived in Phoenix, I'd not be running
15w-XX. At colder temps, much of an oil of heavier XXw is likely pushed right through the bypass.