I'm looking for some help on selecting a lubricant for some pneumatic cylinders.
You have not explained yet why the pneumatic cylinder needs grease lubrication.There are dry pneumatic cylinder seals/ guide combinations used in pressure intensifiers capable of operation at pressures above 10 ksi.
Sprague Gas Boosters | High Pressure Company
Piston-Type Pressure Intensifiers - Womack Machine Supply Company
Your 1000 psi seal requirement is easy by comparison. The seals and guide rings are teflon composite polymers. They can be made to order.
Your oil free 1000 psi nitrogen supply system already makes use of such seals.