I did it for years with no problems. Yes, there are those that say that the oil will form a pocket around the water based and potentially cause an abcess, but the likelyhood of that happening is very very slim. After you give youself a shot just rub it in and you should have no problems. Keep in mind that the water may flow through faster than the oil - so be careful as to not push the oil extremely hard b/c when you get to the water it will squirt out very quickly.
nautica