When I used them, I found turpentine to be the most effective way to get a "paint" consistency from oils. (I can never figure out cross-Atlantic translations, so that may be white spirit, or it may not be). Artists use impasto effects where the brushstrokes remain visible and aren't worried about smooth flat finishes that we need for miniatures. So they don't mind the paint being more like toothpaste than "paint." But oils hold the shape of brushstrokes pretty well, so you'll need to thin them just to the point where they cover, but don't have enough strength to hold ridges or bumps. There are "drying" agents you can add to speed up the drying time, but generally oils will never approach the speed of drying acrylic (water-based) paint. I found that greens, in particular, take
forever to dry -- one time I waited two weeks for a very dark green to dry. On the other hand, oils have a long working time so you can blend and use effects you can't with acrylics.
If you're painting metal figures, you can use a low-heat source to help the drying time. Some use a crock pot, I used a hair dryer set on low, and stuck it in a vise where I would blow it on the figure for 30 minutes at a time. Using a low-heat source like that also helps flatten the paint finish, but you'll probably find yourself using matt varnish (Pebeo makes a great matte varnish). BTW, I learned that oil paints don't actually "dry." Instead, the oil in the paint reacts with the oxygen in the atmosphere to crystallize and harden, fixing the color onto the surface. If I recall right, the correct term is oxidize, which sort of means that oil paints don't "dry," they "rust."
Good luck with the oil painting!