Engine oil

My guess would be someone told them they could gain .00000045% more fuel mileage.

 

Exactly what I was thinking. And when you're getting rid of the truck at 500k anyway, might as well make sure the engine internals look like they have 2 million miles on them while you're at it. People can argue their brand of oil until they're blue in the face, but everyone pretty much agrees that viscosity is important. 15w40 has been the normal dino viscosity forever. 5w40 synthetic is normal, and people argue that's too thin. But 10w30? Maybe if you're trucking at the North Pole or something.

 

Well ####. I was planning on keeping up with the 10w30 because it's what they have been running. Now I'm not so sure.

 

Lower viscosity is easier to pump, and could lead to better fuel mileage. Maybe someone can chime in and tell us why there could be any possible reason to keep running it other than that, but otherwise, I'd switch to 15w40 dino, or 5w40 synthetic ASAP. I wouldn't even wait until the next interval. Just drain and fill. Depending on your miles, you can probably leave the filter alone.

 

This link explains the viscosity.
Engine oil: Oil viscosity index.

Engine oil: Oil viscosity index - Xenum Power of Technology

I wouldn't be too concerned about changing your oil asap.

Check with the manufacturer and find out what viscosity they recommend.

The mega you bought from probably bought newer equipment and didn't want to stock 2 different oils. Your truck was probably backward compatible.
So they changed your oil instead of the new trucks.