My paint of choice is Dulux Trade oil based eggshell. If I am painting a large area and want a flat finish, I add a little bit of white spirit and more importantly, some
Owatrol oil to maintain a wet edge. Post 2010 VOC compliance, I now add a tiny bit of Terebene to speed up the curing process if required.
BTW, if you are painting with oil based paints, spend £20ish on a BrushMate 4 box. At the end of each day, you clip the brush back in the box, put the lid on and the brush is instantly usable the next day. You can fit up to 4 brushes in the box. They are suspended and don't touch the bottom, and hence don't become contaminated by other colours.
The box has a pad inside that releases a vapour which keeps the brushes wet.
BrushMate would rather that you replace the pads when they become dry. I dribble liquid from the bottle for the BrushMate 20 (the bottle is about £9 Vs £5 for a pad). Assuming that 2 pads last a year (at tenner), you dribble liquid from the bottle on to the pad, and you end up with about 4 years.
My 2" brushes cost about £16-18 each. I can use the same brush for years without having to clean it.