Gentlemen - Couple of comments from a guy with a chemistry background about cosmoline, deisel fuel, kerosene, gasoline, laquer thinner and citrus-strip solvents. Len's already posted what cosmoline is - generally a high-boiling, heavy fraction produced in the crude oil distillation process (that's cleaned up by further refining and testing to ensure conformance to specifications).
Because it is an apolar, non-protic material, a good solvent for it needs similar properties. That's why deisel, kero, mineral spirits, laquer thinner, naptha, gasoline and similar non-protic, apolar solvents work best. Citrus strip works by two mechanisms - it is somewhat hydrophobic, and it also can modify certain heavy hydrocarbon chains that contain double bonds to form more water-soluble compounds.
Anyway, as far as health risk, the afore-mentioned hydrocarbon solvents go from low to high in this order: mineral spirits, kerosene, deisel fuel, (naptha and laquer thinner - about equal risk), and way up there, gasoline. Gasoline contains substantial amounts of polynuclear aromatic hydrocarbons, as well as benzene. Both of the PNAs and benzene are potent carcinogens. One could argue about the placement of naptha/laquer thinner and deisel in this list. As a relatively unrefined crude mixture, diesel can contain a fair amount of PNAs. Laquer thinner and Naptha don't, but they have very low boiling point components, so you're likely to be more exposed to those compounds as they vaporize in the air. The health implications of long-chain, low-boiling alkanes are up for debate. Certainly they are less hazardous than PNAs, benzene, and certain sulfur compounds found in low-grade fuels.
I noted this because gasoline is often used as a solvent, and is extremely dangerous. One of its properties that makes it so hazardous is that it has a very, very wide range between its lower explosion limit and its high explosion limit. These are the concentrations in air that will propogate an explosion - too low, and there's not enough fuel. Too high, and there's not enough oxygen.
But there's a special property of components of gasoline that make a fire so potentially dangerous. Diesel, mineral spirits, and kerosene do not have really low-boiling components that will readily evaporate and form a vapor cloud. That means that if you soak some wood with these and throw a match on it, a fire will start, but it will not have much in the way of flash power. In fact, you can drop a match in a bucket of diesel and the match will go out (please don't try this!).
However, gasoline, naptha, laquer thinner and to some extent white gasoline (camping fuel) have lots of components that have a flash point right about room temperature or lower. That means that they will rapidly evaporate and form a vapor cloud, and they are so readily flammable that an ignition source will cause not just a fire, but an explosive detonation. Roughly speaking, 1/2 cup of gasoline in vapor from has the equivalent explosive power of a stick of dynamite. And some of the early naval FAEs (Fuel-Air Explosives) used naptha and compounds similar to gasoline as the fuel source. Fuel-Air Explosives were developed to substantially increase the potential of an air-delivered weapon. They are sometimes called "the poor man's nuke".
In other words, puh-leeze do not use gasoline as a solvent. You may get away with it 100 times, but the 101st might be way too exciting.
