I used to use oil washes but have moved over to the sludge wash method, as featured in a FSM a couple of years ago.
This is a much easier method of creating a wash and what is more, it is easier to remove if all goes wrong.
Use;
5 parts water
2 parts paint colour for wash
3 parts dishwashing liquid
Mix these together but do not shake (for obvious reasons) and apply over the glossed model surface. Leave to dry for 20 minutes and using a moistened cloth, cotton buds and/or micro brushes to remove the excess wash. This will leave a nicely defined panel line or the wash in nooks and crannies.
The key ingredient is the dishwashing detergent which retards drying time of the wash mix. Avoid using black (unless really necessary) as it is too stark a colour which will leave your model looking like a patchwork quilt.
I am a big beleiever in scale effect!!
cheers
Mike