Found this article on oil painting mediums that might be helpful.
I had posted another, which charges fees for reprinting, so I have posted this one instead.
It is from How to Draw and Paint and the link is below.
Have a look here at the main oil painting mediums and a brief description of what each one does.
Gesso Primer It may seem odd to start the oil painting mediums section with what is in fact an acrylic medium. However, it's widely used as a base coat for art canvasses when oil painting. Comments for gesso regarding acrylic paint mediums apply equally to oil painting mediums.
Turpentine The best known thinner and cleaner for oil paints and brushes. Use the distilled artists version rather than the household version for best finishes on your painting. Traditionally mixed 50/50 with linseed oil for an excellent medium. However, its powerful odor is not always welcome in the house and may be an irritant for some artists.
Low Odor Thinners An excellent substitute for turpentine, in all the areas mentioned above - without the smell!
Linseed Oil After turps, probably the best known of the oil painting mediums. On its own gives colors a high gloss. Added to colors it produces a glaze effect. Used 50/50 with turpentine or low odor thinners, it provides a good, general purpose paint medium for oil painting. Slows down drying time. Compared to some oils, it can go a little more yellow over a period of time.
White spirit A cheaper version of low odor thinners and turps. Ok for thinning paints for underpainting, but probably not for quality work. Fine for cleaning brushes.
Prepared Oil Painting Mediums Varying from one manufacturer to another, a combination of white spirit and other oils to provide a ready-mixed, user-friendly paint diluent. A beginner could use this, instead of mixing their own combination of oils and other additives.
Stand Oil A faster-drying version of linseed oil. Reduces consistency of paint and brush marks.
Poppy Oil For adding to lighter colors and white. Less inclined to yellow than linseed oil. However slower drying.
Gold size Although primarily intended for applying gold leaf, it provides a relatively fast drying oil-based paint medium.
Alkyd Gel & Liquid Oil Painting Mediums Alkyd oil paints are well known for their much faster drying properties than regular oil colors. Alkyd paint mediums can be added to conventional oil paints to speed drying time by up to 50%. Can also be used as a glazing medium. Like acrylics, the glazing technique is where a translucent color is painted over another, dry color. The lower one glows through but is affected by the density of the top glaze. Creating misty or smoky backgrounds is a good example of a glaze.
White Alkyd Paint Strictly speaking, this isn't a paint medium, but I use this a lot to speed the drying of conventional oils where I want a lighter tint or a highlight, as opposed to a glaze. The white alkyd paint, when mixed with other colors, acts in the same way as the alkyd gel, but doesn't lose the opacity of the color.
Gloss or Matt Picture Varnish A spirit based varnish, equally at home on acrylics as well as oils. Dries to a gloss finish and will not yellow or bloom. Gloss and matt varnishes can be mixed to give a satin finish. Can be removed with turpentine or white spirit.
Retouching Varnish A thin varnish which can be painted over a touch-dry painting to 'lift' areas where the oil has sunk into the canvas, leaving dull spots. Can also be used as a temporary varnish, say for exhibitions, where thicker paint on a recently completed painting may take many months to dry through completely. Can be removed prior to, or left on underneath, the final varnish coat.
Damar Varnish Dries in a few hours with a satin - medium gloss. Removable.
So there we are. If you've read this far, it'll be obvious already that several oil painting mediums do fairly similar things.
Probably the best ones to start with are Low Odor Thinners, one of the mixing oils such as Linseed oil and one of the other additives that promote quicker drying.
Then as you become used to them, try out the other oil painting mediums one by one. This way minimises confusion over what to use and when and stops you wasting your money.
In addition to this artictle, here is a link to Sander-Studios: Tutorials-Drying Oils and Mediums
Ron Sanders' "Triple Self Portrait" was the winner of the most innovative painting in the National Oil & Acrylic Painters Society 18th Annual 2008 Exhibition.