Rear main seal

Still "Close" to starting used engine for first time... I have primed it, due to it sitting for 3 years. No oil drips on floor, but keep noticing a drop on the bottom of crank. A buddy of mine, that's a mechanic and builds classics said he wouldn't worry about it...that it would most likely seal after engine runs.. Is that possible, or was he just worried I'd ask for help replacing rear main...lol