Flushing/internally cleaning an engine

[Tracker1]

I admit I have never done this, or seen it done, so I was wondering if it is some sort of urban myth. I have heard of diesel and transmission fluid - and maybe there is some sort of singular cleaner for this purpose? I have a high-mileage '78 T/A that I want to avoid taking apart, but shows grime on the dipstick and under the valve covers and generally makes a mess of its PCV system with oily gunk. So If I want to flush it internally, who has had good results and with what method? As always, thanks in advance for your knowledge.