Assembly oil is designed for two purposes one real, the other imagined(FOR US):
Did you use Engine assembly lube? It's thick stuff but It should wash away as oil passes through the engine.
1. The real goal:
To avoid all the "dirt" that a normal oil (that flows) generator. The assembly oil is sticky, thick, and therefore does not flow. This is a feature that is important ONLY in a factory that production of large quantities(Mass production), for the individual user the contribution is negligible to non-existent at all.
2. The imagined goal(For Us):
To stay there(the oil) for a long time until initial activation. But assembling engines at the factory only takes a few hours, and chances are good that the engine is turned on, for the first time check, till 24 hours. Even if it's takes a few months, the sticky oil has NO advantage attributed in terms of "first start". On the contrary, it has the disadvantage that it does not cover all the parts nicely and does not reach every point and dot. Parts that are soaking in 10W-30 oil, like we are soaking clutch pads in oil, it's much more effectively for this purpose (not to cause increased wear on initial operation).
In other words:
Assembly oil is for factories, and to prevent dirt generated from the use of ordinary oil. The small amount of assembly oil creates more friction and abrasion than to soaking parts in 100% engine oil. Soaking parts in 100% engine oil which covers the metal at 100% also prevents rust in a much more efficient way. At the factory some of the increased initial wear is desirable (a matter of adjusting the parts to their place), and in any other use of assembly oil it has mainly the disadvantage that which we ask avoided from (increased wear).