After God knows how many tows (I lost count) it really was the sludge again. They had installed a pressure gauge and oil pressure was fine, but started dropping from 85 to 20 within 10 miles. Suspecting the o-rings in the oil inlet tube, they dropped the oil pan and there it was - the sludge. Got sucked up into the oil pump and clogged everything up. Cleaned everything up with carburetor cleaner. No problems for the last 60 miles. Engine sounds very different though - the click click click noise just got a little louder which makes me wonder what got damaged each time I drove it with no/low oil pressure (total of 4 times, 1-2 miles each time) - ? There's no loss of power, though... 201K on the engine. Am I in for a valve job or worse any time soon? Also, the after market oil pressure gauge couldn't be installed permanently, apparently only Volvo gauges can, and these are supposedly expensive. Any suggestions? Thanks everyone for helping me through this.