I've heard the same things about diesel in engine oil and also in the gas tank to decarbon on long trips. I've also read the deal about "no detergent motor oil" too. Then there was the old "ATF and water in a Coke bottle" trick to decarbon too, but that required some experience to do correctly.
Diesel, just as is ATF, is a thinner oil that does have some detergent characteristics AND will get into places to restore oil flow that normal motor oil might not. It penetrates and then opens things back up with time. Plus keeping all of the oil soluable gunk in suspension as a good detergent motor oil will do too. If the gunk has big enough particles, the filter should catch them too.
As for the "no detergent motor oil", the detergent in motor oils is designed more to clean "as it happens" rather than do a specific quick clean up from existing deposits. There are any number of additives which will clean the inside of an engine that has gotten gunked up from lack of maintenance or just prior to overhaul. In some cases, the more HD cleaners will have you add them to the oil and then run the engine at fast idle for at least 30 minutes before changing the oil. Whether or not you do that with the existing dirty oil or put a new oil and filter in first might be a judgment call the owner made.
When I bought my '70 Monaco Brougham 383 4bbl, when I looked inside the valve cover through the oil cap hole, I noticed some buildup on the rocker shaft. I found some Stewart-Alemite CD2 Oil Detergent additive. I did an oil change and when it got 1 quart low, I put a 1 pint can of the additive in instead of additional oil. When it got back to the "Add" line, I added another pint. By that time, the rocker shaft was pretty clean. I did an oil change and the oil stayed cleaner longer.
One other thing I usually do when I do an oil change is that after the oil is still dripping out of the drain plug hole, I'll pour an extra quart through the oil filler cap hole in the valve cover. Basically, this makes sure that most of the residual oil in the pan is flushed out as soon only new oil, with a thin line of dark oil, is running into the drain pan. Not sure if it works or not, but with the price of non-synthetic oil (even back then), it's a cheap deal to do.
There was also a CD2 additive that was similar to STP, or a viscosity improver, but what I was using was the detergent additive. It caused no problems and did get things cleaned up too. I'd rather trust a name brand than something else for things like that.
ALSO, as everyone has been concerned with oil in the engine, if the engine's been dormant for an extended period of time, it might be a good idea to pull the valve covers and pour oil and some extreme pressure oil additive down over the rocker shaft and also down the push rod holes in the head. It's one thing to make sure the bottom end's taken care of, but that upper end stuff needs oil quickly too (the only oiliing the lifter and cam lobes get is from oil thrown off by the crankshaft, for example). After doing that, then remove the distributor and distributor drive gear so you can turn the oil pump with an adapter and a drill motor. This will get all of the internal oil passages lubed up prior to start-up too, but probably not affect the cam/lifter area that much (other than the cam bearings).
As for an "in chassis clean out", you would need to remove the oil pan, the valve covers, and intake manifold and related items so that you had unrestricted access to the lifter valley and upper end of the motor. Then, one you could use the diesel fuel to flush things out and probably then wash everything down with Varsol (or similar) before putting everything back together with a fresh oil and filter change. That should get everything out of the heads, lifter valley, internal block surfaces, and the oil pan. Messy? Yep! Effective? Pretty much. But it would not address the combustion chamber deposits and such, which can then be done and a final oil/filter change too.
As for fuel additives, it has been mentioned that some fuel cleaner additives can thicken the engine oil. I read this regarding Techron. They are designed to be used one bottle at a time, basically. It was noted that using Techron "double dosed" or one bottle with each of two consequtive fill-ups can thicken the engine oil, so some caution might be in order there.
In any case, resurrecting a dormant engine can be rewarding AND somewhat labor intensive AND not always done very quickly. Of course, probably the best thing to do would be to pull it out and take the whole thing to a competent and reputable machine shop (usually NOT a mass rebuilder!) who might have an appreciation of and expertise with Chrysler engines.
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