yes an oild based wash is usally best, or even an enamel one. if you do an enamel one over the future you can use enamel thinner/white spirit to clean up and unwanted bits and it wont effect the future as its acrylic based.
or you could try ordering som of phil's wash. you can read about it here. its great stuff!
Weathering. - Promodellers weathering wash.
and you can order it here:
Weathering. - Buy the wash.
(please excuse the plug, its just great stuff and so easy to use!)