I think if your engine oil is coming out as clear as when you put it in, then the oil is not doing its job. The oil filter picks up the larger carbon deposits and metal shavings (from engine wear) but there are plenty of fine particles that will pass through the filter. A good oil should act as a carrier of these fine deposits, which should come out when you do your oil change, otherwise it means its still in your engine. I've religiously changed oil every 5000km in my cars, and I've used a variety of synthetic oils including some very expensive ones. I have never had an oil change that has not come out dark brown or blackish. These are cars that myself or my family have owned since new and have less than 100,000km on the engine. Also I'm just wondering, whats made you have such a bad opinion about Castrol?