MX13, now what?

At that mileage, you may be ahead to do a Diesel Force treatment, rather than cleaning all the individual components.

 

Good idea, have done it on my DD15’s
Both at 300k

 

Now do the math. It averaged 33.3 mph.
That tells me a lot of idle time.
400000 miles in 5 years would ring another bell. Possibly a shop queen, caused by idling?

A friend of mine bought a T700 with a MX13 at around 1.2 mill km. It took him about 2 years to figure out the bugs. He likes it.
I myself run one of the most unwanted engines, a 2006 CAT C13. I like it. For me, there might be even a Paccar on the horizon.