FWIW the original requirement was for 20-20W. The bearing clearances were rather tight on the rods so it takes a low viscosity oil to squirt into the dippers and adequately lube the rods. Oil temperature probably tracks water temp pretty closely. So I wouldn't think you'd need a 50wt. multi-grade even in Phoenix.
I've been following Lake Speed's YouTube videos on oils and he does scientific testing for hire. He has pointed out that oil formulated for diesels has high calcium as soot dispersent and that the calcium competes with the ZDDP for the spaces in the metal-surface irregularities. So the net effect of extra zinc (ZDDP) is much less than you'd think. It's complicated but the net effect he says to use gasoline-blended oils in gasoline engines. He has also done camshaft wear tests with various oils and found that flat-tappet cams do just fine with modern low zinc oils of adequate viscosities. The videos are very informative and about 20 minutes or so long. Its a small investment.
I quit using DELO 15-40 when I no longer got it for free and went to VR1 which is a racing formula and suffers viscosity breakdown as it's assumed to be changed frequently, like after every race, says Lake. So then it was Rotella T3, and back to VR1 after I saw the video on diesel formulations. All this on my gasoline powered pickup. Its 235 ran nearly 200,000 miles before it burnt a piston and was retired. It now has a 261 and I'm using Valvoline synthetic 10-30 on the next change in place of T3. We'll see. I'll send in the drained oil for analysis and see what I get back.
