Assembly oil

You can just crank it until it starts. That is a valid aproach. It's one I personally don't care for much, considering how long it takes to get oil up those empty passages to things like rockers and such.

I personally tend to remove the spark plugs and on overhead cam engines losen the valves to non-functional. I then crank it over until I (a) get oil pressure up, and (b) can visibly see the oil flow at the rockers. You'd be amazed at how long b can take.

Then I set valve lash, install the plugs, and and fire it up.

Here's the probelm. A new engine has no oil up in the galleys and it takes a while for the first oil to push out all the air. Those first many seconds are not only critical but really tough on fresh bearing surfaces that have not polished in yet.

Assembly lube is a creamy material that won't drip away. It clings to the surfaces until fresh oil washes it away. Between the heat and washing of the fresh engine oil, nothing reamins.

I'd never spend all that time building a new engine only to have a lot of wear occurr when I first turn the key. Assembly lube is worth the time.