This 32oz bottle of d-limonene costs 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 (orange oil) to thin my paint and make my mediums.
D-limonene is classed as a terpene, a type of naturally occurring solvent. In fact it's extracted from (and smells like) the rinds of oranges.
It's an excellent and safe solvent for oil paints and is extremely strong; it will happily strip away a layer of varnish with the same ease as pure turpentine.
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.
In practice, I've found it to be very effective. The only real difference with OMS I've found is that the drying times for the paint seem to be extended: 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 at Amazon—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.
Lavender oil (also known as oil of spike) is also a good alternative solvent that's available in bulk at Amazon. It's somewhat more expensive than d-limonene—about $40 for a 16oz bottle but it might be worth it to change the studio odor once in a while.
Because of the cost, 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. 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.