My F150 Calls for 10/40. I ran the 15/40 for a few years. The engine has close to 200,000 on it,, I reall dont know, because the speedo doesnt work right, didnt when I got it,, so, it says about 160,000,, now, how many miles have slipped by? I dunno,, anyway,, I was getting lifter clatter.
Not immediately, it took a coupla years,, was it gonna do that anyway? I dunno. I went back to 10/40. I didnt see an immediate improvement,, its still there, not sure its better.

Cold, its sposed to flow as if its the lower number, hot, its sposed to flow like the higher number, SO, its almost always getting what will behave as 40 weight, so how could it be a problem with the lfters? Im not seein it. I just decided to see if I could note an obvious difference,, I cant,,
SO, In answer to your question. Im going back to the 15/40 in the truck. Its In the lawnmower & backhoe. My mower is an MTD 2 cylinder, has a filter, so it has a pump, Its 7 years old & we have a big yard & I abused it by clearing some brush, meaning to walk thru you needed a machete. The area was about 7500 square feet, but I didnt just try to roll right thru it, I just whittled the edges. Took 3 years to win that,,
Anyway, Ive seen mowers like that crapped out, burning oil, no power in way less time.. Ive run the heck out of it & its hangin in there just fine,,truck doesnt use oil at all,,