I go by hours.
Pretty much depends on a lot of factors. That's why there are guidelines from manufacturers. Not sure what large-truck manufacturers say, but I know that large fleet operators depend on lab reports for their changes. And it seems that more manufacturers have smart systems monitoring engines for this very thing: pretty sure that
I'd seen (because I was looking at them) late model Ram Cummins had oil monitoring systems.
One thing that people aren't factoring is TIME. Don't know about others, but I'm too busy to be doing unnecessary work.
Here's an unrelated (to oil changes) example to expound on how one tends to overlook time for things perceived as being menial...
My VW TDIs average around 50mpg. For my car, since I've owned it, I've filled the fuel tank a total of 147 times. If I were to figure 10 minutes for a fill up (aiming the car toward the station, stopping, filling, logging
and then setting back out) that's 24.5 hrs! If I were driving a car with a similar sized fuel tank and getting 25 mpg I'd be looking at an additional 24.5 hrs of time just filling up! Raise your hand if you've ever felt that you'd like to have more time for something! NOTE: my car is actually averaging 51.5 mpg over the course of 103k miles, so my average of 50mpg isn't inflated (but, it does cover for my wife's car getting a bit under 50 mpg!).
My point with the above is that we have a way of overlooking the amount of time we use to do things: sometimes we don't want to admit that something which should have taken only X amount of time ended up taking X + Y time!
Regarding water/moisture, I delved into this issue a bit when trying to track down where my Kioti was getting all the water that was ending up in it's fuel filter ("water in fuel" light coming on): it turned out to be via a crappy/cheap locking fuel cap! During my investigation I was sure it would be my bulk diesel fuel tank: it has a water filter on it; and no water found from the drain. I ran across the following info that I thought was pretty profound:
The Myth of Condensation in Fuel Tanks by David Pascoe: Boat Maintenance, Repairs and Troubleshooting
Water, unless you have holes somewhere, isn't going to be forming in/on your oil. Any issue could only really be that of condensation on metal surfaces (or on a surface that lies between two areas of differing temperatures, which can't happen if everything is acclimated together, sitting statically for periods of time). I suspect that rust isn't going to be happening on oil-coated parts, which then comes down to ensuring that an engine is run frequently enough to ensure that oil is dispersed on all surfaces. When oil is warmed up and chemical activities are occurring is when the oil [additives] is going to interact with any oxidation or minute moisture. IF your oil is healthy then it's capable of dealing with this. Why change the oil? Any contaminants that one could be concerned with aren't happening IN the oil; they happen with the oil absorbing through chemical interaction (flow and heat): oil up against the side of a material is going to effectively block oxidation. Change the old oil and fire it up and what happens? The NEW oil does the same work that the old, healthy oil would have. Most engine wear occurs at start-up. Likely the stuff that ain't neutralized by the oil additives right away is the culprit; but, again, this has less to do with the condition of the oil than of getting the engine up to temp (and getting the viable oil doing its job).
