If you are changing your oil on a regular basis, your engine is staying clean enough as far as the places that need oil
People seem to forget, the oil going into your crankcase has DETERGENTS in it
it is very difficult to find NON DETERGENT motor oil
Your fuel system on the other hand can get very dirty
if you don't change your air filter when it needs it, dirt is carried by the air flow & goes into your fuel system
Then even worse is the fact that the SMOG System pulls oil Vapor out of your crankcase, so that the engine can breathe, these vapors are put into your fuel system, supposedly to burn off when the spark plugs fire. Instead of actually burning off, they only cause the oil to char & build up carbon deposits. Sea foam or a similar product help to keep these deposits at a minimum. Of course a Catch Can will help to keep the oil vapors from entering the fuel system.
For those that have always had a smog system on their vehicles, before about 1968, vehicles did not have Smog Systems, the engines all had a Road Draft Tube, the engine crankcase breathed by allowing the excess pressure to release through the draft tube.