Q: Loud knocking from motor that randomly started while driving down the road at 45 mph.

If the oil is not being consumed, it will turn black due to heat and carbon buildup. If the oil passages were dirty with carbon and other buildup, then the new oil acts as a detergent and will pick up the debris. Excess heat will also cause the color to turn. Different oil types will react differently to heat. Since the oil is not being depleted, your attention should be drawn to the excess heat.

The most likely cause for both the excess heat and the knock is bad spark. Issues with spark usually include a weak or mistimed signal. If the combustion happens at the wrong time, the result is the knocking you are hearing. You may notice the knock at idle due to the lack of other engine noises and a lower frequency of sounds to drown out the noise. Failure of the rod bearings on the crankshaft can also cause these sounds. It is more likely on an engine as old as yours. You are correct that an overhaul is likely required if this is the case, which is why you may want to see if the spark is causing your issue first. You can do this by checking your timing and spark plug condition.

Either way, I recommend having a certified mechanic, such as one from YourMechanic, come to your location to determine the source of the sound and see what repairs are necessary.