In my experience of hundreds if not thousands of oil changes. I can always say Castrol oil seems to always consistently come out blacker than any other oil used.
I have used about every oil I could at one time or another.
Pennzoil, Quaker State, Shell, Valvoline, Mobil 1, Havoline, Super Tech and many others I used full synthetic, semi synthetic and conventional. I buy what's on sale, but I like Castrol the best because I really do think it cleans better.
I have also ran Marvel Mystery oil in the crankcase, although I only added a pint, not a quart as they directed at oil change and ran it the entire 3000-5000 mile oil change before with nothing but a smoother sounding engine.
I've had new cars and my son's F150 , I had and sold him has over 270,000 and doesn't use a drop of oil between changes.
It had several MMO doses. I probably do the MMO every 5 th oil change. Just my observations.