Yes, Yes, and No.
My first car, an LC Torana 138ci 6 cyl.
sludged to the point that the rocker arm movement in the rocker cover was molded to fit. Scraped the muck out of the rocker cover, a sump full of kero, and some idling, and it worked pretty good.
A neighbour and I used a diesel treatment probably a half dozen times over similar years for rattly tappets (hydraulic).
Anything that you are likely to come across these days probably wouldn't be a good idea.
These were all 60s designed engines, with relatively low pressures on cams and bearings, and low specific power outputs.