I have had 1 x VR Exc 1 x VS Exc & 1 x VS Berlina all with the same problem without dramas. The 95 Berlina had this problem from 60,000km(under warranty) until I just sold it at 285,000kms. I took it to a few Holden dealers over the years & it was cause by having more frequent coolant changes due to higher kms & the oil residue is from the pellets added to the coolant every change. If the coolant is not flushed properly the pellets (lube) material builds up in the system. Holden`s put in 3 pellets every change to ensure that all parts are lubed, so they won`t fail under warranty. I just done my first water pump after 280,000kms. I would not worry too much!