Today I thought I would do a little comparison of some of the mediums that can be used with oil paint. These are all alkyd resin-based mediums. They have a moderate drying effect on oil paints. As you can see they all carry a health warning label and need to be used with adequate ventilation, some of them are pretty smelly. A small amount can be mixed directly into the paint, or you can dip your brush into the medium and then into the paint (I always wipe off the brush after dipping into the medium and before dipping into the paint so that I don't use too much of it.) I always get the smallest bottles because the larger bottles dry out after opening, and it lasts a really long time.
Liquin Fine Detail is really watery. I don't like it. It's too easy for me to get too much into the paint, and I have also found that it stays tacky for a long time before it dries.
Weber has videos on their web site that show you what all the mediums are used for, it's definitely worth watching.
I love Gamblin paint, but I don't like their Galkyd medium. It's really sticky. It has almost a honey consistency to it, sort of halfway between a gel and a liquid. It's really gross.
Neo Megilp would be my all-time favorite medium except that it dries slowly. As an example, Liquin may be dry to the touch in 24 hours or so, but Neo Megilp will take about 3 days to dry. I love the consistency of it, it's a soft gel that dries to a nice matte finish, and it's just a dream to work with, but I never use it on models because of the slow dry time. If they could just make this medium exactly like it is, but with Liquin's dry time, I think it would be the perfect medium.
And that's all I've got laying around. All mediums should be used in small amounts, but they do help to dry oil paints a little bit faster, and they also make the paint a little more transparent so you can create nice glaze effects.