When sourcing writing pens, the ink is the most critical component. You should prioritize low-viscosity oil-based ink for ballpoint pens to ensure smooth writing without skipping, or pigment-based gel ink for superior water and light resistance. Request a writing length test report; a high-quality refill should achieve a continuous line of at least 400 to 1,000 meters depending on the pen type. Additionally, ensure the tungsten carbide ball tip is precision-engineered to prevent leaking or 'blobbing' during extended use.