"Refer to paragraph 3 of this link:
http://propanepower.com/3.html
"The purity of the HD-5 grade motor fuel adds longevity and efficiency to forklift engines by eliminating most of the carburetor, regulator, and starting difficulties inherent with the use of commercial grade propane which has a much higher propylene content. Propylene, the main impurity found in commercial grade propane, is the black, oily-like substance which mechanics often find, much to their dismay, clogging and fouling propane fuel systems."
Sorry....have to disagree with this statement. Propylene is typically obtained via two main routes: either as a co-product of the refinery catalytic cracking process used to make gasoline or as a co-product of the steam cracking process used to make ethylene.
For simplicity's sake I will not dicuss the catalytic cracking process for gasoline.
About the other process, propylene is produced in the refinery when ethane feed stock is introduced into a high temp furnace about 1250F with high pressure steam. The main bi-products of what is termed temperature conversion or "cracking" are ethylene (C2H4) and propylene (C3H6). Through a series of towers (fracinators) the products are separated. Ethylene and propylene are feed stock in other processes to create plastic and other products.
In either process the last separation is in a column called a C3= splitter. Propylene (C3H6) is lighter in composition than propane (C3H8) therefore it exits the column at the top and the propane at the bottom. Both propylene and propane at atmospheric conditions are colorless gases and highly flammable. They become liquid when compressed and pressurized about 180# and leave the process unit as a liquid to be stored in tanks.
Any entrained residue resulting from the cracking process would exit the bottom of the C3= splitter with the propane. It is the refinery's responsibility to make sure this material is not transported when propane leaves the refinery. The possibility of finding oil in a propane system does exist but should be rare.
It is not propylene!!!!