Insolubles are a combination of soot and oxidation byproducts, in general. There are some other contributing factors, but I'm not going to try to split hairs at this point. To be "insoluble" in a lubricant sense, means to not be capable of disolving into the host fluid.
It is not uncommon to see results from .2 to .6, regardless of filter brand or OCI duration in "normal" use. (I am speaking of full flow filters, not bypass filtration.)
While Blackstone would infer that acceptable insolubles = good filtration, I'm not as convinced. It's certaily good to have a low insoluble count, but I don't know that one can directly attribute filtration to insoluble levels. It's perhaps more of an in-direct relationship.
If you look over lots and lots of UOAs, you'll begin to see what I mean. Super-premium filters like Pure One and EaO return insolubles around the same range (.3-.6) as a common Fram, in most UOAs. But I certainly agree that the filtration efficiency of a Pure One or EaO is well past any ability of the "orange can of death", as I once heard it called.
I don't believe that one can draw a fair conclusion on filtration from insoluble readings. I personally think it's much more an indication of the generation (origin) of oxidation byproducts than it is filtration; it's not so much a matter of how well a filter "filters", but more that likely it speaks to how your engine system as a whole (design, fuel used, efficiency of combustion, lubricant selection, filter selection) works in concert to produce, or not produce, insolubles. Most filters are 15um-20um nominally, so I don't think it's fair to say that the filter is the controlling factor when insolubles can be well below that size range. The fitler might control insolubles that are large in size, but the presence of insolubles at smaller sizes (say below 10um) must be attributed to different means. A filter only removes what is present; it's the generation of the insolubles at their start of inception that makes me think it's more of an "overall" byproduct, if you understand my meaning. If you have low insolubles, and the filter just isn't capable of catching them due to it's pore size, then it must be the lack of insoluble production, in the first place, that gives the low reading! A filter cannot catch what isn't made, right?
I am open to discussion on this point, as I have no real "proof". I do have a large study going of diesel UOAs, and that is what leads me to this conclusion, however it is not the main thrust of my study; I have not manipulated the structure of the study to focus on insolubles. It's more of an "educated guesstimate" at this point, and I'm willing to take on new viewpoints, should someone have solid information otherwise. I just don't see a direct relationship between insolubles and filtration efficiency. You can buy a great fitler, and your particle count analysis will show significant improvements over a common filter, and yet the insolubles may or may not shift accordingly ...