I would have to agree with fatalifeaten that it was probably a plugged oil passage. But one thing you did say- it was a newly rebuilt motor, did you do it or someone else. and whoever did it did they change out the oil cooler or at least back flush it. If the engine had a bearing failure to begin with and the oil cooler was reinstalled then that could be the cause of your contamination. VW oil coolers are built with lots of tiny passages up through the fins that can trap debris from an earlier bearing failure or just a careless oil change. This debris is almost impossible to clean out. It will then be released back in to the oil flow just a little at a time. You can back flush it as I have done once or twice before and have had some luck... but if you want to do it right it should be changed. On your crank you will most definitely need to get it to a machine shop and have them magnaflux it to see if it is cracked, because even a little heat can most definitely cause a problem.I would also have them check it for hardness also (I'm use to industrial diesel applications) as most forged cranks I have seen are up around 40 to 45 rockwell hardness and on the cast cranks we checked they were around 30 to 35 rockwell hardness. If it were to test out at around 25 or below on the rockwell scale I wouldn't use it. I did check my VW shop manual and there is no listing for hardness on a VW crankshaft in there but I know from experience with low HP diesels that a crank with that low of a hardness will not live. Also a lot of times that information is put in service bulletins for machine shop use and not in a shop manual.
66 Ghia, "Dexter" 76 flat window Beetle
It's not a Car it's a VolksWagon!