I lubricated mine with a synthetic instrument grease that I've had around for about 30 years (don't ask where I got it, I don't remember). Anyway, I like this grease because it never hardens like the natural greases do. Any good high quality automotive grease should work too, but it may thicken over time and need to be replaced again. It needs to be thick enough not to run so it'll stay where you put it, but not hard thick like a wax. Whatever you use, be very careful not to use so much that it works out and starts dropping on your wood (that's the only real critical part of your choice and application). A very thin coat on the shaft, rubbed on with a finger is enough (but you'll probably need to do it again in 5 or so years).
The retaining ring can be obtained from Makita (long wait and probably expensive) or from any "real" hardware store. You know, the ones that have been around forever and have little bins of all kinds of precious stuff. Take your sander with you and show them what you need. They will sell you one that fits (buy at least one extra - the replacement may fly away too). These clips are called E clips (my son calls them "fly washers"). They are the only mechanical parts that I know of that can accelerate to light speed in less than one inch of travel. You never see them leave, but you always hear where they've been. (looking where you last heard them will never locate them - where they stop is a silent landing, usually on the opposite side of the shop from where the last sound came from)