I highly doubt due to BPV. Most elements that discolors the oil is less than a micron. The source of dirt is the combustion w/c is made by fuel and air. Kung hindi maganda ang sunog, it leaves more carbon that contaminates the oil.

Normal lang siguro ang discoloration not unless may sample ka ng old oil at na compare. Try checking din ang air filter for leaks. Kung naka aftermarket filter most probably hindi maganda ang air filtration.