painting over old oil based paint?

hello i need to paint over an old wood exterior window frame, i think the old paint is oil. i have been told the rule is to never put latex over oil (correct?) i will sand and scuff up the paint to the best of my ability. the frame is black so if i use a primer i would need to use a dark primer but as far as primer goes i only see white as a choice? is there such a thing as a darker primer for surface prep when the finished color will be dark? what would be the best way to go about this, and what is the usual method for painting over old oil paint either exterior or interior? also what are the best type brushes to use for oil / to use for latex?