Mobile 1 doesn't make 5W40 high mileage. I am using their 10W40 high mileage too, but change every 4-5k, instead of 2-3k compared to dino/blend before. Switching to 5W40 would improve your gas mileage when engine is cold, but at normal operating temperatures it makes no difference.

Castrol also has a High Mileage 10W40...but it comes in a synthetic blend. I don't think they offer a full syn version.

I used that before, but after learning they can add 5% synthetic to 95% dino oil and call it synthetic blend I got skeptical about it.

Well, I got Shell Rotella T5 15w40 (synthetic blend) out of Walmart for $20 for a gallon. Also got a quart of Lucas High Mileage, but turns out I need 6 quarts total, and local Autozone/Advanced Auto do not carry Shell T5 in single quarts, if at all. In fact, oil selection there and in Walmart is pretty dismal compared to what it was back before Covid. Literally there wasn't really much to choose from. Mobil 1 variety was also very limited, so I pretty much had to chose Shell Rotella T5. Plus it seems that the T5 15w40 is one of the most popular choices for this truck across a couple of forums... Is it the perfect choice? Hell if I know.... It's almost September here in Washington, D.C. area, and this oil will likely still see some driving all the way through Christmas. And it does get pretty chilly here, but not like in Ontario, Canada or even Detroit, Michigan, for that matter.

You'll be fine. I run Rotella T6 5w40 full synthetic year round in the DC area. I think it's a better year round choice, but 15w40 is fine. I also prefer synthetic, as I like to change it every 5k instead of 3k with conventional.

I always order engine oil online. Walmart ships free for orders over $35. They have the M1-301A oil filters too.

Disco:
Good to know. Thanks for the heads up.

To all:
I have a Turner engine that has gone 30K. I have been using regular Mobil-1 synthetic. Added the ocassional zince additive from a bottle. I am wondering if I should change to the high mileage formulation for the additional additives for the valves and rockers. Thoughts please?

The M1 10w40 HM will have more detergents in it as well as lasting 10k miles if you like. I don't see how it can hurt. And I buy my oil and M1-301A filters on line at W-mart. Best price and availability that I've found.

Well, I ended up returning the Rotella T5 15w40, then almost fell for the Mobile1 15w50 full synthetic, but then realized it's a highly questionable choice in D.C. area in September; and at this point I got a jug of Mobile1 10w40 synthetic high mileage oil. So, I gather a few people here are already using Mobile1 10w40 synth HM? At least the 10w40 is among the LR-recommended weights of oil, and it should be ok for autumn and even moderate winter (Washington, D.C. area is fairly moderate during winter, gets bellow freezing, but not too much bellow 0, if at all). Also, with Mobil1 for $24 you get 5 quarts, no 4. So with a quart of Lucas, I only need the 1 jug of Mobile1, while with Rotella I had to buy 2 jugs, since nobody seems to have it in single quarts... I guess if this was Spring, and I was going to drive it in traffic jams and much of city driving during the summer, either 15w40 or even 15w50 could be reasonable choices. But I tend to mostly use this crocodile on freeways and highways and such. I do haul stuff occasionally, but nowhere near the 7700 lbs max rating.
P.S. Installing an STP oil filter, el cheappo replacement of Motorcraft FL300 and Purolator L14670. Are these any good? It says it's rated for 5000 miles, which is how long I plan to run this oil, anyway. Worst comes to worst, the filter can be replaced mid-cycle with minimal oil losses, right?

When engine oil reaches is normal operating temperature it makes practically no difference if you use 15w or 10w oil. It makes a difference when oil temp is way below normal operating temp: 15w has a higher viscosity than 10w oil creating more resistance.