AJ, they are not oils, but watercolours and that's my favourite medium for washes..! I like them best because they are versatile and easy to remove if I feel I've put too much or else (just place the model under the tap!). Sepia is the colour I use most for washes. To get the best results, you'll need to add a drop or two of dish-washing liquid and of vinegar to your mix. Let me know how you fare.
Domi