Sorry to bring up such an old thread, but it seemed more sensible than starting a new one. I have similar 'problem'. I never notice this prior to bottling, but in the bottles I can notice a small oil slick on top of the beer, maybe a dozen or so droplets. When I pour the beer, these droplets spread out across the surface of the beer. My beers tend to have poor head retention, and I am inclined to blame this on the 'oil' on top.
In a previous life, I used sodium metabisulphite, and do not recall ever seeing these droplets of having such poor head retention. I am currently using iodophor. A lot of other things have changed since then. I am now doing all grain, and using a plastic watercooler as a mash tun. I used to use mostly extracts, with a small amount of grain.
I can't see how any of this stuff could make a difference, which is why I suspect the iodophor. However, I have kept beers for almost two years in the bottle and it has not disappeared, so if iodophor is meant to be metabolised like you guys say starsan is, then it can't be the iodophor.
If it is hop oils, I'm fine with that, but hop oils aren't meant to affect head retention, so the poor head retention must be due to something else.
Anyone got any ideas?