Originally Posted by Breckman99

Most likely it is the over boost pressure switch. It tells the switchover valve to stop delivering boost to the alda therefore cutting the supplemental fuel flow. It is a system safety, so if not working and you have an over boost condition there will likely be damage. If it is leaking I would pull it out, clean up the threads, and put a good automotive grade thread sealant on the threads to see if it stops. That "oil/fuel" looking stuff is what has built up inside your intake manifold due to the EGR and crankcase ventilation system.