Plastics and Chemical compatibility
Avoid ... "EP" gearbox oils, Synthetic oils, Ester based oil, Polyglycol based oil, any oil containing additives such as Molybdenum Disulfide (Moly) or graphite.
Excerpt from http://machinedesign.com/mechanical-drives/engineering-essentials-lubrication-tips-plastic-gears-and-more-part-2
Q: What affects the compatibility between lubricants and plastics?
A: Compatibility factors include the lubricant’s chemistry (base oil, thickeners, and additives), viscosity, and aging resistance.
Chemistry. Typically, lubricants based on silicone, PFAE (perflourinated), most synthetic hydrocarbons (SHC or PAO), or mineral oils work well with plastics. Lubricants based on esters or polyglycols are generally not compatible with plastics, although there are exceptions depending on the type of plastic.
Incompatible lubricants cause plastics to lose dimensional stability or structural integrity, or become discolored. To check for compatibility, manufacturers test physical properties of the plastic material including volume, weight, elongation, strength, and hardness. Each manufacturer sets limits on the allowable change in these material properties, typically 7 to 10%. In evaluating such tests, be sure they reflect your worst case conditions. Both lubricants and plastic materials are more prone to changes at higher temperatures or in adverse environments, especially with high dynamic loads.
Additives sometimes cause a lubricant to react with plastic. For example, solid additives, such as graphite or molybdenum disulfide (moly), can penetrate and weaken a plastic component and should generally be avoided. On the other hand, PTFE solid additives are useful in specific cases such as reducing startup friction or providing dry lubrication.
EP additives used in lubricating metal parts are not recommended for plastic parts. Moreover, large amounts of corrosion protectors or metal deactivator additives used with metal parts are also unnecessary for plastics.
Recommendations. Mineral-oil-based lubricants don’t attack most plastic materials and offer excellent performance for the dollar in general plastics applications.
The eagle eyed amongst you will have spotted that some synthetic oils seem to be ok with plastics, so why don't we recommend them?
Its mainly for environmental considerations. Although no less toxic than a mineral oil in many respects synthetic oil is designed to last longer in your engine without breaking down (some manufacturers claim 3 times longer) so it follows that they will take longer to biodegrade in the environment than a mineral oil.