Oily Aircon drivebelt

I have a slight leak of oil on the offside of my A4's engine ('96 1.8 20v ADR) which I noticed the other day. The aircon belt had been affected - the 'flat' side becoming slightly concave. I cut off the (poly v) belt and was glad I had, as there were signs of chewing where some of the ribs must have been riding up in the outer grooves of the pulleys. On the following weekend I replaced the auxillary belt (which wasn't old but had rough spots on the back, perhaps where oil had been flicked up at it?), degreased the engine, checked that the cambelt was dry (via the upper access cover) and stuck an old aircon belt I'd been carrying around on as a replacement. ( I had intended to leave the aircon non-functioning, but when I found I couldn't get the auxillary belt off of the crankshaft pulley working from above, without releasing the aircon idler pulley - necessitating that the front of the car come off as per usual - I thought I might as well have continued use of the A/C.)
I've been keeping an eye on it for the past two weeks and noticed today that oil is once again finding its way onto the replacement aircon belt. Now the engine is cleaned, it looks like it comes from a point just above where the aircon compressor attaches, but I can't eyeball it directly.
Has anyone here come across this issue (the car has done 180k, btw). Like I say, the timing belt seems to be dry, which has to be a blessing...!