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⁾

Caution You need to turn them upside down and then back right side up a few times to get the oil to drain from the inner chambers. Do not over fill them, they could blow apart. Put on one shock at a time and make it bottom out and force out all excessive oil, that is still too much because the inner air chamber will still be too small. The only way to tell how much oil is in them is by weighing them. There are several different years and models of shocks and no one knows all the specific differences. ³⁾