Which type of brush filament is best for oil-based paints?

1. Price: PET is the cheapest material to made the flat paint brushes, pure PET can be made in big hollow. Flaggable type. PET hollow type always in disposable type.
2. Tapered PBT/pet filament can be made out from  PET+20%PBT; PET+30%PBT; PET+50%PBT or full PBT.
3. Filament for paint brush diameter suggestion: 0.15-0.25mm around, changes along with the length of filament.
4. Solid taper synthetic filament can increase the strength and elastic, paint brush filament with taper tip can absorb more paints. Fan shape means hollow in the middle, picture as below: this can increase 40% paint absorb in one brush. Mixed with solid, increase the whole brush elastic.