If you have a board contaminated with oil a car wash (the kind where you use a pressure wash and do the cleaning yourself) can be a good move. I've had to do this a couple of times when customers put weird 'contact enhancers' on their tube sockets which leached out over the entire board.
A soapy pressure wash followed by a long water-only pressure wash did the trick. Of course the board has to be allowed to dry completely and you likely will have to pay attention to certain areas of the board. Contaminants and water can collect under tube sockets soldered to the board. So sockets will likely require special attention (I've not had to remove them). The board should be placed vertically so water can drain out.
I've tried various chemicals but had only limited success. The pressure wash has worked quite well.
