-Is the 15 PSI oil pressure what is being measured at idle while the engine is hot? If so, this could be normal. If you’re getting only 15 PSI at higher RPMs than that, not good.
-Is the 93 PSI that the mechanic says the engine should have, an accurate number? It sounds awfully high for a passenger car engine.
-Has the oil been changed regularly?
-How was the sludge determination made?
-Finally, what made you suspect internal engine problems? Does the engine make mechanical noise or clatter? Is the oil light coming on? Does it lack power?