Hello Orville and Welcome!
For oils, I find that I use both firm and soft brushes. This is just me, but I like to use bristle brushes for underpainting, and the softer sables and mongoose brushes for overpainting in color.
I use a variety of sizes and shapes, but I have my favorites. I love a good flat brush - there is almost nothing I cannot do with it. Also, a #1 round is great.
I can recommend the Isabey Mongoose brushes. I have nearly the whole set and I really like them. They are firm near the ferrule, and quite soft near the tip. It makes them springy enough to do deep spreads of paint while remaining soft enought to blend with. I use them only in the color stages. My ground has tooth and wears down brushes, so I would rather wear down a $4 bristle brush than one of these.
As to natural vs synthetic, I find that I don't like pure synthetics for oils, but have several blended synthetic/natural that work very well.
Hope that helps!
