2004 Pontiac Grand Prix Lower end failure

The 3800 motor in my 2004 Grand Prix started rattling last weekend while I was on a trip of 250 miles. I took it to a local dealer and they found metal chips in the engine oil so it appears that something loosened up internally. About 1000 miles ago, I put synthetic oil and a Mobil One filter in the engine. The mechanic that is doing the engine swap claims that switching to synthetic oil likely caused old oil varnish or other particles to get between the crankshaft and bearings, causing this failure. Have you ever heard of this? I can't find any information suggesting that synthetic oil should not be used in an engine that has run 80000 miles on convetional motor oil.