sounds like there might be a head gasket problem or somewhere combustion is getting into the oil,i.e. if the head or block had a crack in certain places it could be forcing compression into the oil pan, but pretty unlikely, most of the time when you have a prob. with head or gaskets it will either fill the oil with water or vice versa, drumsy has a point, it would be more likely for the dip stick to be leaking at the block and dropping oil onto the exhaust and smoking up that way, but you sound pretty convinced it is coming from the dipstick tube, if not i would start with a compression check,