Oil paint leaves the tube thick. The first layers are made leaner with a small amount of white spirit or turpentine. Later layers take a painting medium such as linseed oil. The rule is fat over lean: a more flexible layer over a leaner one, so the film is less likely to crack. Buy a few colours you will use, and buy brushes on their own. A set that bundles weak paint with poor brushes wastes the first weeks.
