For most 20-year-old KitchenAid Blenders the grease packed inside begins to break down into UCK and oil which will drip through the old gaskets.
After breaking down the case and removing all of the old grease and oil I cleaned all the gearing and shafts. I replaced the case gasket, the 2 shaft gaskets that the lower orbital gear slides up, and then the o-ring on the main shaft after everything was installed.
I did a break-in run on med speed for one hour and then tapped the chrome ring off of the lower unit and flipped the blender over and checked the 9 mounting screws to make sure they were still tight.
Reinstall the ring and the blender is ready for the next 20 years. The Kitchen Aid site as well as others will give you the parts diagrams, as do many other parts sites. It's good to have a copy for future reference.
Here at Doc' Shop we use APPros parts on many of our jobs and have always had successful job outcomes. They have good pricing and FAST shipping.
Good luck with your KitchenAid repairs as this is surely a DIY job.
Doug