This 32oz bottle of d-limonene cost about $34 on Amazon.com
In early January I came to the realization that I couldn't go on using petroleum-derived solvents like odorless mineral spirits (OMS) in my studio. Even Gamsol and other highly refined versions of OMS proved to be strong eye irritants for me (mine are unusually sensitive), and regular low-odor mineral spirits was making me feel ill. Painting outdoors or with the windows wide open in summer isn't a problem, but in the winter months when the window is open just a crack it isn't possible to properly vent any fumes.
After doing some research I decided to start using d-limonene to thin my paint and make my mediums.
Despite anything you may have read elsewhere online, d-limonene is an excellent and safe solvent for oil paints. It is generally considered safe to the point that it is used as a food additive, yet it is extremely strong; it will happily strip away a layer of varnish with the same ease as pure turpentine. d-Limonene is a terpene, a class of naturally occurring solvents, in fact it's extracted from (and smells like) the rinds of oranges.
It is added to things as diverse as chewing gum (for flavor), natural cleaning products (as a solvent) and is also used in cosmetic products and some alternative medicines.
Unlike OMS, of course, it has a noticeable smell. That may be something that you find bothersome at first, but I found that I got used to it very quickly. There are actually benefits to using a solvent that has some odor, in particular you know when it's on your hands.
It is a very effective solvent for oil paints. The only real difference with OMS that I've noticed is that the drying times for the paint seem to be extended somewhat. d-Limonene appears to evaporate more slowly when mixed with paint than OMS does.
Price-wise it costs substantially more than OMS. I paid about $34 for a 32oz container - about 3 times the cost of OMS from the hardware store, and about 2 times the cost of a quality OMS from an art supplier.
Because of the cost and odor, I decided not to use d-Limonene to clean my brushes. I've started to used safflower oil from the grocery store instead. At supermarket prices safflower oil costs about the same as OMS. It's completely non-toxic and can be used in any regular metal brush washer pots. Disposal requirements for the contaminated oil are the same as for OMS where I live in Seattle, so no change there. The only drawback is that it doesn't get your brushes completely clean without additional washing in soap and water - and you need to pay special attention to the roots of the brush hairs near the ferrule to make sure all the paint is out.