I would change the oil more frequently instead of adding any atf or magic potions. Zero risk of causing damage unless you twist off the drain plug.
Pull the valve cover and look for dry rocker arm (doubt it but maybe). Are any rockers loose? Turn the crankshaft two turns while watching for a rocker moving less than the others indicating a flat cam lobe.
Yeah, I really am learning of flushing it due to age and oil leak potential. But will running an oil without detergents in it clean out sludge deposits in the lifters or other essential areas?
It might just be rockers in the end, the sound is louder when I open the oil cap.
Would a a flat lobe be noisy? I would think it's limited movement would be fairly quiet? If there is a flat lobe, then time to move up my cam swap plans.
