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Simply for the fact that these additives make a stronger surface tension which traps in air. Not to mention strengthens the buble's tension of air that gets trapped under the surface. You dont want to have airpockets in your oil!
Most regular synthetic motor oil has antifoaming blended in to prevent this, but when you add something like an additive that wasnt formulated to be in the motor oil in the first place the oil may become unstable.
FOR EXAMPLE:
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