Does oil get darker because of dirt?

With my Chev 6.0 LS I find the oil will be dark brown on the dipstick by 5,000 miles, which means it’s black coming out into the pan. This happens to coincide with when I change it. The new oil always looks light brown the the dipstick, even if I leave the filter on for a 2nd go around. The oil in the filter is not enough to darken my new oil to any significant degree in my case. This engine has over 212,000 miles on it. YRMV. :)
 
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If you have environmental dirt in your oil then you have a serious air filtration issue. Oil gets dark in color from heat and suspension of combustion by products (carbon, soot).
 
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I would say albeit totally anecdotally, it can help it get dark.

When I was young and an idiot, I ran cone air filters (K&N style) on my daily driven cars. I found my oil was super black by 3000 miles, even in a relatively newer engine and in old ones, too. As soon as I stopped using them on our cars, I'd find at 3000 miles the oil would be basically bronze colored, and with 5000-6000 mile oil changes even with conventional/syn blend, it would be less brown than my cone filter oil at 3000.

If you look up some oil analysis on here of people who run cone filters, some have readings like 30-50ppm silicon, when a normal reading is about 5-10ppm, so I'm guessing 5-10x-ing your silicon would make your oil darker. Of course conventionally oil gets darker due to engine heat and detergent cleaning stuff, but I personally experienced my oil get way darker way faster with a freer flowing air filter compared to better filtering one, so it's one reason I'd never put on a daily driven car again now.
 
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Suspended dirt and combustion byproducts will turn the oil darker a bit, but most of the darker color is oxidation and breakdown of the oil and its additives. Cleaning of deposits/varnish/sludge will also contribute, obviously.
 
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