I hope everyone recognizes this thread's original question was posted in 2005. And I sure hope they have not continued to have these problems.
The "oil" in an LPG system is a byproduct of the temp. It is from a molecular/chemical/heat reaction that "cracks out" the oil from the gas molecules at temps above about 130f, but the higher the temp, the more oil cracks out. (that is in part how we refine oil). The "LPG thermostat" goes into the coolant line _out_ from the regulator to the engine, not the line -in- to the regulator. it closes at it's rated temp (usually about 160F), and opens at lower temps. This allows the expansion of the LPG gas to cool off the regulator to a temp less than the 180F+ that the motor coolant is kept at by the engine coolant thermostat.
Since it is a physics/chemical property of heat that causes the oil to "crack out" at the molecule level, I would doubt that any additive would actually eliminate it, although I can see how a solvent might wash it through the system, however, I would want to see the regulator manufacturer's opinion of how save it is for the seal and other parts of the regulator before I added it to any of the units I care for.
Like they say in the car commercials, -your- mileage may vary.