Diff is how strong a core is. Surface is still king, but for a ball to work on heavy oil, you need a strong core, which is represented by lower RGs/Higher Diffs. High RGs mean the ball will lope more before the core starts to roll, and would anyone use the Dart core (diff of .015) on a heavy oil pattern? Even paired with a heavy load particle cover, the core will not face up well enough on longer patterns or patterns with more volume.