Couple2 of random notes...
As far as the K&N air filters are concerned, they do indeed allow a lot of dust and dirt to pass through. It's not quite as critical with a gas engine, but you cannot have air too clean for a diesel. Dust and dirt is comprised chiefly of silicates. Silicates are what causes a bare leather strop to polish the edge of a knife, and silicate is the primary constituent of sand (like, sandpaper) You have to seriously consider whether you want such an efficient abrasive entering into your turbocharger.
The factory fill for the Promasters is Pennzoil Platinum 5w20 full synthetic. There's really no good reason to run anything else. If you want to use another brand, as long as it meets MS-6395 spec, you should be fine. Something to keep in mind, though, is that different brands will cause the engine to wear slightly differently, even if the oils have the same specs. Good rule of thumb is to find the right oil (whatever the engine manufacturer recommends), and the stick with that oil forever, only substituting in a pinch. So, pick an oil and stick with it. Regularly changing brands (or worse, viscosities) will cause the engine to wear weird, causing premature wear and play in the engine components.
Black oil means the detergents in the oil are doing their job. It doesn't not necessarily mean the oil has broken down and is no longer providing the proper lubrication. Some vehicles (Sprinter) use the oil sensor to measure a multitude of conditions, including the electroconductivity of the oil itself and will tell you when to change the oil. Instead of measuring a bunch of variables concerning oil quality, the Promaster only measures the engine duty, such as highway driving, driving in various temperatures, start & stop driving, etc. Lots of short trips and idling will cause the oil change indicator to alert you sooner than lots of long distance drives will. The blackness or electroconductivity of the oil in a Promaster is meaningless, because the oil will need to be changed long before those things become an issue. In addition to the engine using the oil to lube the chain and cool the pistons, it also uses the oil in part, to control the valve timing. So it'll tell you to change the oil much sooner than you'd expect out of a synthetic oil. Instead of 15,000 to 20,000 miles, it'll alert you to change the oil at somewhere between 8,000 and 10,000 miles.
You can't really go by an oil analysis in a Promaster. You could pull a sample at 10,000 miles and the analysis could come back saying the oil is good for another 5,000 or 10,000 miles. But if you go past 10,000 miles on an oil in a Promaster, the chain won't be getting the proper lubrication and the valve timing ill start to be off a little. There are couple of passenger cars that require more frequent oil changes for that same reason, but I can't recall which ones they are.
Plus, the oil pump is variable displacement pump, so it's very sensitive to oil weight and viscosity. It goes without saying that you shouldn't use anything other than the recommended 5w20 oil, but if you go past 10,000 miles on a change, the changes in the viscosity of the oil will negatively impact the oil pump.
In summation, I think you're on target with how anal you're being with fluid and filter maintenance. I think you could trust the oil change indicator a little more than you are, and hen you get antsy about the oil, give it to 8,000 miles anyway. The indicator will come on and you will have about 1000 more miles before you really should change it. But you can comfortably go those 1000 miles before doing so.
