I'm guessing the best weight for breaking an engine in is the recommended weight oil. I believe for a 302 it would be 10W30.
I personally like Mobil because even under hard use i've never had it airate and it usually lasts longer than other oils that i've used.
Good luck.
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previous name: 84stangLX
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT