Steve, Thanks for sharing that info. Bill Creevy discusses using a liquin or wingel (a gel like form) with oil pastels in his book on Pastel painting. It does work with OP (as do virtually all mediums meant for oil paints). The real question is its long term stability. This is a constant problem because if people wish their work to be permanent and durable and if they are going to be selling it then they, IMHO, have a moral responsibility to make certain that the work is accomplished with materials and techniques that to the best of current knowledge on art materials will result in a durable work. Durable in conservation circles implies a minimum of 50-100 years without likelihood of any noticeable change IF CARED FOR PROPERLY.
Pat is correct that the jury is out on various fixatives for OP. W&N even recommends that paintings done with their Oilbars be framed under glass as the wax added to the oil paint alters the durability of the final film.
Doing experiments such as you tried are very useful, especially if they are then tucked away and periodically examined to see what happens long term.
While there are many methods of "accellerated aging" to test art media, all have some problems because to accomplish accellerated aging means exposing the media to amounts of energy (such a light energy) in a short period of time that is equal to the energy they would recieve in, say, 100 years.
Consider testing safe sound levels for workers. What would we expect if we took the sound energy a worker is exposed to over 50 years and exposed them to the eqivalent amount of sound energy over a two week period. Would we conclude that even the quietiest environments are dangerous?
This is a somewhat hyperbolic example but meant to demonstrate the problem of testing and setting standards for high quality art materials.
Bottom line continues to be with OP that all the most knowledgable people in conservation recommend if you wish it to last, frame under glass.