Taking a wild guess here, until the Track gurus chime in, I'd suggest that since you're using the 5.0 for medium-heavy oils, AND since the Rule is close in overall performance to the 5.0 (No, they are not the same, just close, AFAIK), I think the GP2, with its apparent large range of flexibility, would serve your purposes as a heavy oil ball.
Talk to the gurus (Carl, Rick, Clint and Anthony) about how to drill it for your release, and take comfort in the fact their "guess-timates" are about 95-99% accurate.
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Bowling: Just like hand grenades and horse shoes, you only have to get close.
Life: Deal with what is.
CharlesT