How can i clean an engine?

[EJ Ortiz]

Hello

I recently bought a 1997 200SX SE-R and the engine
looks a bit dirty inside its not clogged since plenty
of oil comes out of the oil tubes in top of the cams
but everything inside the engine looks dark brown i
supose the previous owner did not change the oil too
often i check SE-R.NET on using the STP engine flush
but i've heard it's not safe to use it since it can
clog some of the oil system. What can i do?

E.J.
1992 SE-R
1997 SE-R