Break-in oil is designed to help with initial part wear, lifter to cam break-in and piston ring seating.
Usually there is enough slop in the lifter to block fit that some oil will seep past. A shot of oil around the lifters before bolting on the cover doesn't hurt.
If your engine has hydraulic lifters priming them before starting is a must. With solid lifters it's not a requirement, but is good practice.
