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I have heard that when switching from petroleum oil to synthetic that gaskets may begin to leak which did not leak before. I have had somepeople tell me they had that problem using Amsol. I have not had that problem but I began using Amsol at 5200 miles after an engine kit installation so several gaskets became new then.
As JosephJ said, you might run into that on a high mileage motor when the sludge is cleaned off. And it used to happen years ago when the synth formulations were different than they are now and not compatible with some seal materials.
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