prevents oil in the intake. PCV systems on OHC cars will always cause oil to be sucked into your intake. that reduces octane in your fuel and is otherwise generally sub-optimal. You can run a more aggressive tune without pinging if you get the oil out of your intake air and if you run a wet nitrous kit then there's no reason to be without such a kit.
There is no minimum or maximum mileage to start using one but the earlier the better.
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