Which handles heavy oil better?

Wait.  Are you asking No Rules (as noted in the header of this thread) or No Rules Pearl?
On oil, this isn't even close. No Rules.
If you meant NRP vs. Code Black, on oil, you would need to add some surface regardless of the ball. CB is more angular and the cover is not as strong as the NRP cover. So probably NRP

I just took my no rules pearl to a heavy oil alley.  I'm a stoker, who throws around 16.5 mph. was just open bowling so no practice, I pulled it out the bag and shot 246, 9 spare followed by the next 8 with an open split in the 10th.  However by that 9th frame, the shot was going away.

Instead of the lane breaking down, I was pushing oil down my line with the no rules pearl.  By the 2nd game, it was damn near impossible for me to get any solid reaction down the lane (granted some of that was fatigue due to the amount of games I had bowled this weekend).  its beautiful on weaker oil and a fresh pattern but I had a lot of difficulties getting reaction during the 2nd and 3rd game I bowled.

As for the question, I haven't thrown a code black but from what I've seen of it, the No Rules Pearl gets into a sooner break so I'd imagine that'd be better for heavy oil.

I also throw it with a stock finish, I'm sure if it got taken down a bit it would be great in heavy oil.

I meant an original no rules.  Not the pearl. 

Absolutely