It depends what exactly you are trying to achieve. If its just a panel line wash or a wash around raised detail then yes, you'd want to apply a glosscoat first.
If you want to apply something like a dot filter, exhaust stains or streaking then its better to have a flat or semi-matte coat to give the oil something to grab hold of.
As far as the ratio goes I can't tell you exactly but with a wash you really want to have what amounts to very dirty thinner. If you had a pea sized amount of oil paint then a couple of squirts of thinner of of a eyedropper should do the trick.