Linseed oil is the commonly used oil for formulating oil colors. However, other vegetable oils, like poppy oil and safflower oil, work better to extend the drying time of oil paints.
Safflower oil is entirely non-toxic and used as a binder in oil paints to increase the flow and drying time of the colors. If you are working on a big painting project, you might consider using safflower oil as a medium to increase the work time.
On the other hand, the poppy seed oils slow down the yellowing of the oil paints over time, which can be a problem if you’re using linseed oil. They also lengthen the drying time of fast pigments to avoid early shrinkage of the oil colors. Hence, many paint manufacturers prefer using safflower oil or poppy oil to linseed oil.