If your using Road King shocks, here is a way to change the oil in them.

Removing oil: Install the valve stems with the valve cores removed. Connect a piece of vacuum hose between the valve stem and a partially collapsed plastic bottle. Install only one shock upside down. Sit on the bike to force the oil out. Raise the bike to draw air in. ¹⁾

Installing oil: Add the proper amount of fork oil in your weight of choice to the partially collapsed plastic bottle. Install only one shock right side up. Sit on the bike to force air out. Raise the bike to draw oil in. ²⁾