You're mechanic was right. If you swithch back your engine will blow up, your tires will go flat and your transmission will fail. Your windshield might crack, too. Worse than that, your mother-in-law will move in and your're dog will bite you. And if you switch to Penzoil your engine will fill up with white foam and you're engine will blow smoke. And if the IRS gets wind of your switch back to conventional oil you will be audited to explain what you did with the money you saved and did not report.
Find a new mechanic. Don't let idiots work on your car.