use petrol to flush it (IMHO) as it's volatile and will clean out easily, and then run through with water. be sure to make sure it's completely dry before refitting water in your cylinders isn't a very good idea i think

if there's oil in your IC, there will probably be oil in your inlet tract as well, might be worth cleaning out the inlet pipes as well if you can be arsed :P

just my 2¢