A dilutant is a liquid which will thin down your paint so
as to make it easier to handle. Most common is turpentine, but gasoline, so long as it is purified, can be used.
You can dilute your color with oil, but this not only tends to make the paint greasy, it can also make the paint wrinkle and crack. When using turps to dilute your paint, it is a good idea to add a few drops of oil into it to allow for any breaking down of the binder. If too much turps is used to paint with, the paint could become powdery and flake off.
Driers to speed up the drying rate of the paint and so allow for quicker working, are usually made from varnishes and, as in the so-called McGuilp medium, varnish and linseed oil boiled with lead. The varnishes that can be used with paint are mastic, copal and dammer. Varnishes are also used to touch up a picture if it has gone dead or matt and for overworking. This type of varnish is known as a retouching varnish and is perfectly safe to use so long as you retouch when the paint is dry. It can then be worked over with fresh paint.
Mastic, copal and dammer are used as a final varnish over the painting when the painting is absolutely dry to protect the completed painting from dust and damp, etc. Salesmen will advise you on how to choose a varnish for final varnishing and directions are always given on the bottle. If you are doubtful about using them, there are some good synthetic varnishes to be bought which are foolproof to handle. Winsor & Newton's 'Winton Varnish' and Rowney's No. 88, clear picture varnish, are two of these.
The addition of a drier or a varnish to your paint is a matter of personal taste. A medium like McGuilp, or its modified, manufactured counterpart Maroger Medium, helps to facilitate the drying of the paint and gives it a nice buttery consistency which is delightful to handle.
An excellent medium is compounded with half oil (pale drying oil as it is termed on the art store's lists) and half mastic. This forms a jelly which, when agitated, becomes liquid enough to add to the paint. For further information about this medium, see Maroger's The Secrets of the Old Masters. Authorities disagree about the durability of using driers with the paint. But for a beginner, I would go so far as to recommend using a drier as this will help him overcome the initial difficulties encountered with a slow drying paint.
If you do not get on with them, you can return to using just the paint with turps. But don't be afraid of experimenting with them. It all adds to the excitement of painting and will help you to get the most enjoyment out of it.