To properly dispose of old used motor oil you can take the used motor oil to a recycling place such as Walmart, Autozone, Oreillyauto or other auto parts stores and they will usually accept the used motor oil for free to recycle.
Some auto shops may also take the used motor oil for free and properly dispose of it for recycling or they might even have a waste oil heater in their shop where they would use the used motor oil to heat their shop with.
My friend has a waste oil heater in his shop and he uses used motor oil in his waste oil heater so he doesn't need to pay for heat.
He changes peoples oil and people also bring him used motor oil so he uses it in his shops waste oil heater and the heater puts out good heat.
You can also check around and ask your county or city if they have a drop off place for disposing of used motor oil as some landfills have places where you can dispose of the used motor oil for free.
You can usually bring the used motor oil in sealed containers and give it to someone at the landfill or they will direct you to a location where they'll be a table or something where you can sit the used motor oil on and they will then recycle it.