If I create a Lights factory and unlock xenon lights it will fill all the slots with directional xenon lights and not make any other lights which then clogs it all up.
This is an issue with all manufacturing. If you are producing multiple types of vehicles all with different types of engines then you will run into this issue.
When you unlock a new light, engine type etc it gives you the unlock pop up and if you press upgrade all like I usually do it will upgrade all your manufacturing units to only produce the new advanced engine. This is a problem if you have a budget range etc as it will not manufacture engines there now only the upgraded type if you upgraded it unless you delete it to down grade it.