Normally I change my oil, Mobil 1 Syn 10-30, every 5k; lost track the last time and changed it after 8k+ (combined city/highway driving.) Wandering what would be a save amount of miles to change synthetic?

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).