I recently bought a BMW 323i and replaced all the fluids in the car. As I was going through the oil though, I decided to give it a kerosene flush and then changed the oil. This new oil I ran was Napa 15w-40 Diesel oil and I put in a whole bottle of Auto-RX and ran it for 3000 miles. At the end of this interval, I did another kerosene flush (mixture - 30 minutes at idle). Both times the oil came out extremely dark brown and vaguely reddish. This time, however, I needed to do the rinse cycle. I found a deal at Advanced Auto Parts for 5 quarts and an oil filter for $20 + one quart. I decided to buy Castrol GTX 5w-20 oil to give it a shot mpg wise. I am extremely light in my driving style and you will usually see me driving at or slightly below the speed limit in the right lane so I tend to think that I am very easy on the oil. I always accelerate slowly and shift early so the engine rarely goes over 2400 rpm. My question is whether anyone has ever run 20 weight oil specified for a car whose manufacturer specifies 30 weight oil? Ford retroactively specified 20 weight for all cars post 1992 and I think this may have implications beyond marketing and getting cars to self destruct so their owners buy new ones. If Ford cars 1992+ have greater bearing clearances than my 3/2000 BMW M52TUB25 then shouldn't 20 weight oil fit the bill perfectly for my car? Just food for thought. I am interested in what people have to say about this. Let me know if I am not clear about something.