I am planning on doing an Auto RX on my car, as I have heard so much great press from this site, and my car just crossed the 300,000 km mark (almost 200,000 mile) so I want to give my car a present. My question is my car is only 4 years old, (yep is always on highway) and has always has Castrol Syntec in it, with 10,000 km/6,000 mile changes on it. I have had the valve cover off and isn't very dirty under there, no sludge or anything. Wondering if under these circumstances if I need to do the 2 bottle flush, as car has over 100,000 miles or if one bottle will do as might be a special case.