Water in oil is immediately obvious. It doesn't turn it brown in the manner you're describing. In any real quantity, it turns the oil into more of what looks like chocolate milkshake. Oil turns darker in use over time due to particulates and carbon that build up in the oil and are suspended. Fuel and often small amounts of condensation (mostly engines that are frequently started for short durations or vehicles left outside in the weather with a lot of large temperature swings) are also going to be found in oil in higher concentration over time. It is a fool's errand to run the same oil for too long, IMO.chocolate milkshake is brown,