Strange oily contamination on film

I recently got hold of a Paterson orbital processor to develop 4x5 film. I put small blobs of silicon on the bottom of the processor and left it a couple of days to cure. Tonight I developed four sheets of FP4+ film as follows:

150ml LC29 @19+1 for 8 minutes
30 sec stop
4 minutes fix at 4+1

Wash for 5 minutes.

First of all when I opened the processor I noticed that there were milky white areas on the film so I prepared a tray of fix and refixed and washed. That cleared the film but I noticed a strange staining. From the emulsion side it looked like brown stains but from the reverse it was almost like an oily film. As these were test shots I wasn't too bothered about damaging the negs so I gently rubbed the film with a sponge and the oil started to come off staining the sponge brown. I then looked in the bottom of the processor and could see small areas of the oily substance which had the same look as oil on water.

Could this contamination have come from the silicon or the processor? In addition I'm surprised that 4 minutes of fix didn't clear the film. It could be that the fix is getting a bit old but I used the same bottle last week without any problems. Is 150ml sufficient for 4 sheets?

Sounds a bit like dichroic fog. The old fix reacted with the developer and left some complex silver or sulfur compounds on the film surface. Fix is cheap, don't overuse it. Better yet do it one shot in those small quantities.

Gary, forgot to mention that everything was one shot except the stop. What is dichroic fog? These were not uniform but looked whispy.

Ok, I looked up dichroic fog and it looks as though this is the problem. I have a suspicion that the stop bath could have been a bit shot. From what I've just read the developer is not halted before it goes into the fix and this can cause dichroic fog. I would think that this is compounded by the relatively small volume of chemical used in the orbital processor.Time to order a new batch of chemicals.