I've recently started a model project (souping up a Visible V8!) and one of the first things that came up is... with what do you lubricate it? Ask a chemist "what lubricants attack polystyrene?" and the answer is something close to "Oh, wow, just about everything."
The Visible V8 manual advises using vegetable oil, which is a great choice only if you want to sell a brand new model every six weeks. Polystyrene slowly dissolves in virtually all natural oils, and vegetable oil also polymerizes into a solid gunk in the presence of oxygen.
The Internet is full of completely ludicrous advice on the subject of lubricants and styrene. And, of course, vendors don't want to tell you what's in their products or what their criteria is for declaring a particular lubricant 'compatible' with a given polymer. MSDSs are useful for establishing that several lube vendors are probably, to put it bluntly, lying through their teeth about compatibility.
So, what to do? We test.
I've partly immersed two expanded styrene foam peanuts (the very thin skin of the expanded bubbles should show damage well) and one strip of transparent 'crystal' styrene to a number of different lubes and liquids.
In this test, which I expect to leave running for a few years, we have:
Contestant 1 (above): Soybean oil. Sort of a control, because it should be solid useless gunk surrounding trashed styrene in the space of about two months.
Contestant 2 (above): 3-in-1 (black label). Another 'oil' that is usually the worst possible choice for lubricating anything. I expect it to be only marginally better than the soybean oil.
Contestant 3 (above): 3-in-1 (blue label). This is the more serious '3-in-1' that contains a paraffinic oil actually worth using and less of the magic secret crap that makes the black label stuff such a bloody menace. That said, it's a natural group I oil and so should also attack the styrene long-term.
Contestant 4 (above): white mineral oil. Highly refined/purified Group I/II natural oil, also expected to attack styrene long term though more slowly.
Contestant 5 (above): Synco "Super Lube" synthetic lightweight oil without PTFE. This is explicitly listed by the manufacturer as having 'excellent' compatibility with polystyrene. However the MSDS indicates that it contains a substantial fraction of normal mineral oil (approximately 80/20 mix), which means that it is neither fully synthetic, nor is is compatible with polystyrene. We'll see what the test determines.
Contestant 6 (above): NyOil II (Nye Lubricants #123S). This is a product of Nye Lubricants that has legendary status as a super-lubricant on the Pinewood Derby circuit. The MSDS lists the contents as 'proprietary' and 'PAO base' with no additional information. Also, Nye lists the compatibility of the PAO base with polystyrene as 'fair' where other manufacturers list it as 'excellent'.
Contestant 7 (above): Nye Lubricants #269. This oil is recommended for plastics and appears to be ~ 100% PAO. Perhaps just a slightly thinner version of NyOil II? No way to know.
Contestant 8 (above): Labelle #108 plastic-safe oil. Another oil that is legendary as the only thing safe / worth using within a specific modeling community, this time the model train people.
Contestant 9 (above): Amsoil 100% synthetic 0W20. In the interests of trying out something relatively cheap-in-bulk, I've added the only motor oil still explicitly claimed to be entirely Group IV (PAO, diester, etc) synthetic. For the record, 'fully synthetic' motor oils are allowed to be up to 30% Group-III non-synthetic in the US, so other 'synthetic' motor oils are not necessarily substitutes. I believe in Europe, anything claiming to be synthetic does actually have to be 100% group IV+ synthetic.
Contestant 10 (above): 1.5wt Airsoft Silicone oil. Very thin. Silicone is probably the only thing 100% safe forever on polystyrene, so it might seem like a no-brainer as it's safe and cheap. That said, it actually kinda sucks as a lubricant, it dissolves silicone gasketing material, and it tends to contaminate workshops. The problem with silicone oil is nothing sticks to it, aerosolized droplets get everywhere, and it doesn't evaporate so it stays put in inadvertant places forever-- if you use silicone lubricants in a workshop, you can probably forget about trying to paint or powdercoat with reliable results in said workshop ever again.
Contestant 11 (above): Feser One (not an oil). This is actually a non-conductive fluid sold for watercooling loops in computers. I have a use for it (it's the right color for antifreeze :-) and want to make sure none of the additives will hurt the styrene.
Contestant 12 (above): Thermochill Ec6 (not an oil). See 11 above.
Contestant 13 (above): WD40. Sort of an anti-control-- this should be an utter disaster with styrene. In fact, I think it was already dissolving the foam peanut by the time I took the picture. In any case, WD40 is not a lubricant and is in this test only as a warning to others.
...and several compartments left empty as controls, plus one filled with regular water.
...In six months to a year, it will be time to check!