By the way, after a bit of testing i couldnt tell exactly what makes the sack produce sap faster/slower. So, can you tell me what makes it produce more or less?
Its pretty simple - script count brightness level of leaves in square 5x5 from sack position to +12 blocks. Script does not bother - if light source is artificial, or it is sunlight.
More leaves in that range - more rubber tree sap. More light level - more rubber tree sap.
About missing "R":
It seems, that Gregs code somehow transform liquid name from registry. Probably it cut 6 first letters (expecting fluid.rubbertreesap or something).