Paxman linkage oil is a specialty oil formulated to keep the rods and joints which connect the levers to the valves on a French horn (or other rotary valve instrument) running smoothly.
Also available:
Paxman bearing oil 20 ml
Paxman rotor oil 50 ml
Modern French horn rotary valves require three different grades of oil. From thickest to thinnest they are: bearing oil, linkage oil and rotor oil. Apply the first two each week, if you play regularly. When a valve become sluggish, drip a tiny quantity of rotor oil directly into the rotor casing, having first removed the valve slide to allow access.
For reference, a maintenance schedule recommended by the Rosehill workshop is included alongside the product image(s).