M1 0w-40 is not an energy conserving oil. I think you are correct in that it will yield a very slightly lower fuel consumption than M1 15W-50. Not sure if this difference will noticiable however.
Having said that, the 15W-50 used to be an approved oil and factory fill until 0-40 came out.
I think either oil is quite fine. If it were me and I was in Texas, I would use the 15W-50 to handle the wretched heat you all get there.
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Rich Belloff
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