I'm not sure about this, but it seems that mineral oil doesn't turn black faster than synthetic oil.. I've been using these 2 types for years now and .
I was wondering why this oil gets so black so soon?. Unless you are losing coolant I wouldn't do a UOA, but synthetic oil will begin to clean up .
The oil in your engine maintains a thin, lubricating film on all metal parts that lets them glide over each other safely and efficiently, minimizing friction.. If it turns dark or goes black quickly, it's not good.. with non-detergent, non-synthetic oil.
I was told that synthetic oil turned dark much quicker than conventional oil. The dark/brown color was normal and did not indicate dirty oil.
If you do check it and have concerns, here is a guide to what some of the colors may indicate.. Oil can be very, very dark (black even) and still be effective.