I'm posting this because it applies to how cleaners in motor oil may clean varnish and other hard deposits. Varnish consists of polymerized material. One thing it answers that I suspected is that solvent molecule size affects the ability to dissolve a polymer. That is why a very low viscosity solvent can dissolve things so much better than a base oil of with similar polarity but with much higher viscosity and molecular mass. The large molecule size limits the oil's ability to diffuse into the material. Smaller is better at penetrating into the polymer. The paper describes the layers that form in a polymer when exposed to a solvent. A gel layer forms next to the solvent. I found it interesting that the paper discusses how cracks can form in the polymer from internal pressure due to solvent diffusion into it. We've seen chunks break lose from deposits which I bet are a result of cracks first forming.
http://www.eng.buffalo.edu/Courses/ce435/Koenig03.pdf
Here is another good paper: http://doc.utwente.nl/70803/1/Koenhen75determination.pdf
http://www.eng.buffalo.edu/Courses/ce435/Koenig03.pdf
Here is another good paper: http://doc.utwente.nl/70803/1/Koenhen75determination.pdf