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My FIL ran Royal Purple in my car and changed the oil every year since the car didn't get driven 3k in a year anyway. He is telling me to still use Royal Purple (which is fine) and still change every 3k anyway. He's also one of those old quirky guys who just likes to stay busy and that doesn't make much sense to me. Thoughts? Every 5k for that engine (4.6L) for a Mustang GT (15k miles)?
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