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Synthetic oils are superior due to the refinement processes and ingredients used, which result in better scientific control over molecular size and purity. More uniform molecular size means less friction. In your car “used motor oil” is a lot thicker and sludgier with conventional oil than synthetic oil given the same amount of oil wear. Synthetic oil is by far better for performance because it protects the engine from wear due to lower levels of friction and reduced engine drag from greater resistance to thickening. Additives help keep the engine clean of deposits and fewer impurities mean it burns cleaner, thickens far more slowly.