Juniormodeler pretty much nailed it.
When I do a wash I use Winser Newton Oils with Turpenoid (which is a Turpentine substitute). I usually thin the paint with the turpenoid by about 90-95%.
As for Acylics, I pretty much use the same ratio, but I do not use liquid dishwahing soap. I try to keep a semi glossy sheen on the basecoat when I plan on using Acrylic washes.
All in all, I prefer Oil washes over Acrylics, because you have more working time with them, and Oil paint has a much richer pigment.
Cheers
Rob