I recently sold a '91 Chev Z/71 truck that I logged 197K miles on, all with various dino oils changed at around 4k miles, the engine was still very clean inside and still ran great. I would agree that sludge is more a result of mechanical problems or design issues than the fault of the oil itself, not that I disagree with Jellies statement of synth being superior in sludge reduction, and perhaps may be able to overcome minor mechanical or design problems, for most people this isn't reason enough to use synth, not that there aren't some very good reasons for using it.