my Rochelle has really greasy hair, same spot on cardboard when i removed her. it has to do with the type of hair they use. not all of the dolls have this problem tho. i wouldnt worry about it. washing her hair in dawn then googone seems to help.
sticky hair is a concern with some hair, because that is glue seepage. but greasy i wouldnt worry about because its normal.
as for Robecca and Opperetta i have not noticed any greasyness with them.