I've only tried baby oil so far for oil pastel painting, I find that it softens the pastels and it's pretty cool.. though the oil could leave a spot. I've heard mineral spirits and turpentine work as an oil pastel blender as well, and I'm wondering how they work differently compared to baby oil? Would they also leave a stain spot on paper? Would they (or any other alternative) make oil pastel painting drier (less smudging) over time?
Baby oil/mineral oil will never dry by evaporation like turpentine or mineral spirits ( AKA naptha , VM & P). It will not polymerize like boiled linseed oil or plain linseed oil. It may be drawn into the paper or canvas which could appear to be drying.