air bubbles in oil

[Ben Klumper]

Hey guys,

I am near completion of my 3B Coupe Quattro conversion, but I seem to have run into an odd problem. The low-mileage donor engine was completely torn down (except crank and pistons) and refreshed with new seals, gaskets, hoses, etc., including a metal head gasket and urS4 windage tray. This past weekend - after sorting out a hall sender problem - I fired it up for the first time since completing its installation in the coupe. It started up first crank and settled nicley into a steady idle. However, almost immediately, the oil pressure warning light came on in the Autocheck. I shut it down and checked the oil, knowing it would be ok because I just filled it! But the oil was a little strange. It was kind of thick and frothy and looking closely at it, it was full of tiny air bubbles. Now this is different than the way oil looks when a head gasket has blown. It's not milky or creamy, just thick and a little frothy. A couple of other bits of background info: the temperature at start-up was about -5 degrees celcius. When I filled the oil, I poured in 4L, however, the dipstick read that it was slightly full, even though oil capacity is 4.5L with filter. I figured this would read correctly once the oil circulated through.
So, my questions are, does anyone know what can cause these air bubbles? Has anyone experienced this before? Should I be concerned? What sort of things should I check? Could this have triggered the oil pressure warning?

Thanks in advance!

Ben