The shim-based Intiminators inherently use thinner oil also. It's not completely universal, but the lower weight oils listed at
http://www.peterverdone.com/wiki/?ti...spension_Fluid seem to have higher VIs in general (indicating less viscosity change based on temperature). Based on what I know about motor oil, I would also assume that synthetic fork oil would also have a higher VI and be less susceptible to temperature changes.
If you're using a higher weight, non-synthetic oil in your forks and it works perfectly at warm temps, I would expect it to have a higher degree of extra stiffness in the cold, compared to using a lower weight, synthetic fork oil. Just switching to a high-VI synthetic version of your existing oil might be enough to give you more consistent performance on colder days.