I have a CNC lathes that use "oil" for cutting fluid during the machining of mostly metal materials. Typically a heavy paraffinic based cutting oil (petroleum distillate) is used during machining. My company switched suppliers of this cutting fluid and went to another cutting oil, supposedly with compatible chemistry. This happened months ago. Within weeks of making the switch the oil (the two oils mixed together) became slightly "sticky" or tacky. This negatively affected the machine tools and processes and the decision was quickly made to go back to the original product from the original supplier. Over a period of several months the reaction between the two products has left a byproduct of build-up of some type inside the machine tools. I'm thinking it's the paraffin separating from the oil (somewhat) and along with other components of the chemical make-up that's left this brown, sort of sticky, almost "glue-like" material all over the internal surfaces of the machines. I'm looking for some solvent(s) that will quickly dissolve this material. Acetone seems to work but by itself it's not a great option. The fumes are a problem, it evaporates too quickly. Any other ideas?
Naphtha, paint thinner or mineral spirits are common names for the type of solvent suggested by @NotNicolau, and should be effective. Caution: all are flammable and could be ignited by a spark from machining, from a motor or from a switch. These solvents are considered only mildly toxic, though.
Chlorinated solvents such as trichloroethylene are less flammable and even more effective at removing this "varnish" (probably not paraffin, which would be greasy but not tacky) but they are more toxic and the fumes are more annoying.
Ventilate the area well both during cleaning and afterwards until all solvent has evaporated and blown away.