I believe what you're experiencing, in terms of the sporadic shifts in its blending ability, is due to the amount of "wet" paint left in a certain area (For Example). When you apply a heavy dabs of paint with the Oil Brush tool (so "Loading" in the high percentiles; "Thinners" in the low percentiles) and then set the Oil brush to have 0% loading and low to 0% thinners, dragging the tool over the thick paint will result in smearing. Drag the tool over dry pigment marks or marks made with dry media tools (chalk, pencil, etc) and you might get a slight smudge effect, but it won't drag the color nearly as much. This is also the case when you've used the Oil Brush tool (still set to a 0% Loading) to blend an area of thick paint already, because the thick "wet" paint you previously smeared with it will have been thinned out AND dried slightly due to the 0% loading of the Oil Brush being introduced to it.
Personally, I like to use the Pastel Tool (Wax version, set at default setting) for smearing areas. It means having to select a starter color, but it's worked for me.
Nothing is easy to the unwilling.