I'm using baby oil to blend, but Gina K. has posted earlier about the different grades of mineral spirits. Here's part of what she said.
Originally Posted by Gina K. Designs
As far as grades, most companies, including Gamblin, produce several different grades of Mineral Spirits. Depending on the way it is distilled, the evaporation rate will be different. The short explanation is- the faster the evaporation rate, the more dangerous it is. Usually hardware store OMS has a fast evaporation rate as it's meant to be used in a well-ventilated area or outdoors. Artist grades evaporate much more slowly.
Just an FYI.