Comparing base oils of similar polarity, the solvent power decreases as viscosity increases. Solvent power is interrelated with lubricity and additive response. The Hildebrand solubility theory tells us that anytime the average molecular volume increases (higher viscosity), the aniline point is increased and thus solvent power is decreased.
Solvent power and lubricity are interrelated as both require the presence of polar molecules. (Lubricity requires both polar and non-polar molecules to be present simultaneously) Lubricity, in the terms we're talking about, refers to the oil's cohesion to metal surfaces and slipperiness of the film on that surface. Therefore, lower viscosity base oils (of similar polarity) have greater solvent power for dissolving sludge and grime, while also having greater lubricity and thus surface cohesion, assuming the whole oil formula accounts for it. This also makes it better in terms of seal conditioning and dispersancy.
Looking at PAO base oils of different viscosity, just to give an idea...
Aniline point of PAO 2 = ~100°C
Aniline point of PAO 40 = ~160°C