Why is my oil black?

[SquareOne]

Mine will stay VERY clean for the first 5-6000mi. Then gradually get darker. At about 7500-9000 it begins to show a caramel - darker caramel coloring. This last change I let it run out the longest (about 15k and 305hrs), and it was definitely dark, but not black by any means. Could've likely let it go another few thousand comfortably, but just went ahead and changed it .....I bought the engine with 260k from a large fleet (salvage truck), so no telling how it was treated. It's a 2350...BTW with very little idling.

- Now, I can absolutely tell the audible difference with fresh oil. Engine sounds tighter and generally quieter across the RPM range.

- And Hammer, if you think Waterloo's weight will make you chuckle, my rig is only 29k lmao! That's truck/trailer with about 3/4 tanks of fuel (12k of that is the trailer)...its just a daycab Cascadia. Haven't made it to big boy status lol.