Medium Question

So it sounds as if you are simply trying to make the paints slightly looser and easier to manipulate rather than drastically alter the handling properties in terms of unctuousness, stickiness, etc. There are many simple options that would work for you but all would provide a slightly different feel under the brush. Alkyd mediums are fine but they generally are made to add gloss and speed up drying. You sound like you want to keep a longer open time to allow for paint manipulation making them probably not the first choice. You could just add a tiny amount of solvent but that may or may not be too drastic of a change in handling for you. A sound procedure would be to add a small amount of a drying oil mixed with solvent (say part1 drying oil to 1-4 parts solvent like odorless mineral spirits, depending on the viscosity that you require) would be suitable. Linseed or walnut oil could be used and even stand oil would be perfectly fine thinned with even more solvent. Stand oil does add a sticky quality which may or may not be desirable depending on the handling properties that you are looking for. Experiment, you may find that the addition of any solvent creates too loose a feel for you and you could just add a very tiny amount of linseed, walnut, or even stand oil to your paint. The important thing there would be to minimize the addition of oil so as to not create a paint that will yellow too much over time. Let me know if you have any other questions or if I somehow missed an aspect of what you were asking.