Per the 550 Instruction Manual:
Use 30 weight motor oil on the main shaft and bearing grease on the pivot pins.

Periodically wipe the primer slide with rubbing alcohol. This removes the spent primer residue that causes the slide to drag.

Clean your powder bar about every 500 rounds. Some powders build up and will eventually stick the powder bar. Paint thinner, acetone or lacquer thinner works well to remove any buildup

Other than cleaning the powder bar, the manual doesn’t say how often to clean the primer slide or greasing the main shaft/pivot pins.

When do current owners of the 550 perform periodic maintenance?

Are there any other PM tasks to perform to keep your Dillon in tip-top shape?

I don't deprime or prime on 2 RL550's, so I can't comment on how often to clean the primer slide. What I would suggest is when you see it starting to drag (slightly) or when you see enough build up to look like it should be wiped down.

I keep my main shaft oiled well enough so that after 50 to 100 cycles, there is a slight amount of oil that has been scraped off by the housing and I need to remove it. I never let my press main shaft get "dry". I do this on all my presses and they look excellent. And my RockChucker is 40 years old.

Pivot arms need to be lubed before they gall. On my first 550 the pivot arms weren't drilled for lube. I must have thought it was good for life. Well after several years of use and several more in storage I managed to gall one of them. So I learned how to disassemble the press. I lubed them well when I reassembled the press. I make sure I add grease 1/year. Next time I have either press apart I am going to drill, thread and install grease zerks on the arms. It is difficult to get new grease into them as they are now.

On a 550, 500 rounds is a little over a hour of loading. I've come off a loading spell that is around 8000 rounds, and did none of the mentioned maintenance. I can grantee, that on the powder bar, wet lube is a MAJOR problem waiting to happen! On main rams, I avoid any wet lube as it attracts grime. I use One Shot case lube. It is a wax that sprays on an drys right away. On the primer bar, it has been about 75,000 rounds since I touched it, and I am sure it needs cleaned. It sounds like the zerks fittings might work, but watch out for grime getting in the grease and making things worse! I think I would go with a teflon or moly lube, and have only the thinnest coat possible. I have a case feeder on my 550 and that gets wax lubed with a quick squirt about every 600 or 700 rounds during a lull in loading to fill primer tubes, powder measure, bullet tray and case feeder, I take a snack/bathroom break all at the same time. Canned air and small paint and tooth brushes are your friend, and I keep angled tweezers at hand also. I've been toying with the idea of a penlight on a arm to light the area too. Ivan