The exterior block archetecture is similar for the 302 & 351. Outwardly the difference is in the deck hieght. The 351 is about .79 inches higher as measured from crankshaft centerline to the top of the cylender deck. The easiest way to tell is the distance from the distributor hole boss to the top of the block. The 302 boss is almost even with the "china wall". (front intake manifold parting line). The 351 is about .75" higher. There are other ways to ID it but not without some disassembly. The attached is a 1969 351W block stroked to 427. Hope this helps.
As far as the oil is concerned it would not matter about what oil you use if the motor has more than a few hundred miles on it. After break in I use Mobil #1. Lots of guys use Castrol synthetic.
Allyn
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