All our RC vehicles come with oil-filled shocks. These RC shocks have a removable top, which allows you to add or replace the shock oil. It is normal to lose some shock oil over time, and you may want to change the viscosity of the oil, to help tune the suspension to the type of surface you will be racing on. Additionally, replacement/spare shocks usually do not come with any shock oil. The viscosity of the oil ( along with the piston design of the shock) will determine how quickly or how slowly your RC Shock responds to bumps. CPS is the expression for dynamic viscosity, or absolute viscosity of the oil, and is the measure of the fluids resistance to flow, and is a standard way to rate shock oil. If you want to shock to respond quickly , you want to use a lower CPS shock oil. If you want the shock to respond more slowly you use a higher CPS oil. For instance for rock crawlers, you would want a fast response to bumps, and you may want to use a lower CPS such as 350. If you are driving on smooth surfaces, you may want a slow response to bumps (and less chassis roll) and want to use a high CPS shock oil such as 600 or 800 RC shock oils. RC Shock oil is also rated in "weight" however this is less standardized and varies a bit by vendor. 350 CPS oil would be in the 25-30 WT area, 500 CPS Oil would be in the 40 WT area, and 800 CPS oil would be in 60 WT area. For most vehicles, a medium viscosity ( 450 to 600 CPS) is a good place to start.
There are a number of good videos on YouTube on how to change the shock oil