[rlh930]

If oil has leaked into your heat exchanger (the most likely cause of the smell), e.g from a leaking oil return tube or from any number of other possible sources, you will have a devil of a time getting rid of it. You can stop the leak but the smell will not go away quickly. Your best bet is to remove the heat exchangers and clean them somehow. You will be amazed at the amount of oil that will drain out. If you can't take them to some place that cleans industrial parts, maybe you can wash them out at a coin-op car wash