Oil Pickup Modification
Early versions of the oil pick-up had a deflector welded to them. This caused cracking in the tube, leading to air bubbles in the oil. Later engines had a separate deflector. You can ameliorate this problem by changing two crankcase nuts to the later type and bolting the early deflector to them with rubber dampers. (you have to hammer the deflector a bit to shape it as needed. This is an O.E. mod to some engines).
See also http://www.citroensmaustralia.com/maintenance/OilPressureDrop.htm for information about the oil pump and pickup.