First look for any indication of a problem; checking the soot, silicon, fuel, and oxidation (oil thickening). Excessive amounts of any of those can relay a problem. Your engine has something like 750k miles and while it could easily go twice that before needing an overhaul, it's enough miles that things could start to fail, or at least lose efficiency.
Right now, it appears that your engine is as healthy and efficient as my 400k mile Detroit. As long as it remains that healthy, you can safely add many miles/KMs to your intervals. Did you have to add any oil? How much? If you're pouring in gallons of oil for top-off, then that would skew things a bit. I overlooked "Shell" earlier, so I would guess that it's good ol' T4.
If the oil thickens much out of grade (over 16.3 for a 15W-40 I believe) then it may be time to change, or at least getting close. Just my opinion, but maybe not scientifically accurate. Caterpillar labs measure oxidation. If you were to get a sample of fresh oil tested and then test the subsequent used oil samples, I don't know what the condemning number would be for oxidation.
Excessive soot that overwhelms the oil's ability to disperse it will cause extra wear. If you have a bad injector (or two, three, etc), maybe a problem with EGR or the turbo, you could develop high soot and eventually high wear. Just something to keep in mind as your engine piles on the miles. Keep track of how the engine behaves, the fuel efficiency and the visuals to help keep yourself ahead of any problems before they get out of hand. Oil analysis is an excellent tool for finding problems early on.
Once you push the oil too far, it's going to show up in increased wear metals. One thing that many people go by is the TBN/TAN relationship. The theory is that once the acid number (TAN) exceeds the remaining base number (TBN), the oil is done. I'm in no position to say that's wrong, but I've never seen proof of it in my own results. It's certainly not something I can make a blanket statement about in either way, but I don't worry about TBN/TAN too much anymore. If I pay extra for those, it's mostly just for fun or pure curiosity.
Edit: to be more direct in answering your question about what values to look for; the closest I can get to that is the viscosity increase I mentioned above. And that's only my opinion. There are most likely condemnation points for the different wear metals (each engine model a little different), but I don't know what they are for the MX13. I highly doubt you'll ever reach any condemning numbers unless something goes wrong. There's probably a condemning soot number as well, just don't know what it might be. The oil is capable of handling quite a bit but I'm at a loss and couldn't say just how much at the moment. I'm way past my bedtime and need some sleep.