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I have witnessed that once you use synthetic and switch back to dino oil the motor will go. One of my friends used sythetic in his honda since the day he bought it, when the fuel started to get high he figured he could save money and switch back to dino oil. Around 6 months later he motor started knocking. He brought it back to the dealer and they told him the bearings were going out. Luckily they replaced the motor under warranty.
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You observed a problem and came to the wrong conclusion.
You can switch back and forth, go half and half and the oil will never be the problem. If you switch a high mileage engine to synthetic be prepared to put you hand on the oil filter, often, and change the filter when it's colder than the engine (plugged).
To clean out a gunked up engine, use Marvel Mystry Oil, first. Bad engine, use a quart, run it 1k miles and change oil and filter. Simple.