When I buy any car with an older engine and/or I am not sure of the service history, I will do a few quick interval oil changes right out of the gate.
In you case, you might go to Wal Mart and get their syn oil. Run it 3000 miles three times. Assuming you drive a decent amount of miles, this will perform a safe "flush" without the risk of your engine detonating.
I do the same with auto trans. I am NOT a believer in the flush. I simply drain and refill more frequently than the manual says. This also seems to work well.
As Lil said, flushing is NOT without its risks.
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