why dont people change engine oil

[flyerncle]

In the days of the CVH oil was one grade, 20w/50 usually and the carbon would collect every where as the detergent in the oil was not too good and did not break it up.
I have seen engines with a sump full of black tar,one a Mondeo and the other an American motor home and you would think someone had filled it with tar as there was a tiny amount of oil still around the pick up.
Be carefull with modern flushing agents as they can sometimes do more harm than good in older engines.