They are not stress cracks. They definitely look more like oil/lube etc. Also, who would spend the time/money to do a rebuild/refresh on a block with gross cracking if that's what that was? You'll be fine.

Stressed is a under statement, I've seen a block shaken like a dog $hitting razor blades when a couple of guys I know get around a block with a wrench in there hand.

Yes assembly lube. Thanks.