Ditto - what brand of grease? Assuming it is compatible with most auto paints and won't contaminate the finish. Maybe you have a Sata part number for it?
So the old guns (Binks) my Grandpa cleaned professionally had a 'packing' the needle went through. Is the 'bushing' you described it's replacement? I do grease my needle in that area because I know it goes through some kind of SEAL be it packing or something newer just wondering if the bushing you describe is the new version of a packing. The old packings weere adjustable for tightness and grip just like a valve packing....
Do not use that. If you use that your paint is gonna came out full of fishaye. Use professional grease from sata or iwata any brand is alright but spray gun greaser free of silicon