Done just that... ran 3 lt of oil through it... nothing found... filled it and ran to hot and let it sit a couple of hours, took a sample and dropped the magnet in again, nothing.

I have topped it off and will do similar a couple more time after driving until I get confidence nothing still lurking.

Thanks for the advice guys..

(ps I don't what the equiv of Marvel Oil is here, but I can buy some on ePay and give it a try)