Polyisoprene may be soluble in castor oil to increase the viscosity. This material is very viscous and adheres to everything but may be the compound for you to add to your formulation, a little goes a long way!
A good resource for polymer science in this area is The university of Southern Missisppi http://www.usm.edu/polymer/ Dr. Robert Lochead http://www.psrc.usm.edu/lochhead1.html is a leading authority.
Please let me know your results .
Oliver (Jay) Toigo III CHMM, M.P.H. Candidate
320 Cherry Blossom Dr.
Churchville, PA 18966