Read the can/bottle, and instructions should be there.
If you want a quick flush, the best I know of are made by AMSOIL and BG.
The others are not as effective. Generally, you warm up the engine, then add the can of flush and fast idle for 15-20 minutes. Then drain.
If you have more time, check out Auto-Rx. Takes a couple thousand miles to work, but have seen many good reports on it.
Far as oil viscosity is concerned, the first numberX is related to resistance to flow at cold temperatures (or how fast it will pump up), while the second number indicates the viscosity range at 210 degrees F.