1969 Plymouth Barracuda Fastback 318

At 400,000 miles or is it 4,000 miles? If at 4,000 miles 5(or10)w-30 with a zinc additive poured in as well. At 400,000, I'd pull the motor and take a good look inside. If everything looks good, I'd do a basic bearing change and new gaskets with a new cam calling it a day. Same oil either way.