Synthetic glycerin is made by a chemical process. "Natural glycerin" is made by the saponification of triglycerides in fats & oils. Unless you are a tree hugger the two are virtually identical. Not much chemical glycerin made now with all the bio-diesel sources. Silicone and glycerin are two entirely different materials - I would guess many silicones and glycerin will separate on standing.
Most anti-freeze contains ethylene glycol, chemically similar to glycerin but different. A significant amount of glycerin in ant-freeze would increase the viscosity too much.
Glycerin is very water soluble. Seems like it would disappear quickly if the rubber is exposed to any significant water (like rain).
