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How can I best clean, replace or lubricate this fairly well sealed digital rotary mechanical encoder?

The equipment operates, but the encoder is super finicky, often encoding in the wrong direction when spun. With patience it's possible to get it to the right value, but quite tedious.

The equipment is about 15 years old, and the encoder has been unreliable for the last year or so.

Question 1: if it can be replaced, how would I find a pin for pin match?

Question 2: if it can't be replaced, what's the best way to clean or re-lubricate it? It has a crack, I can probably flood some chemical junk into it.

Update: I opened the cleats:

By the limited information you give and the (not too great) photo, it seems it is a mechanical encoder, not an optical one.

You don't give any information about the equipment it is mounted on, but 15 years of continuous operation may be quite a lot for a mechanical encoder. Probably the contacts have worn out and there is no reliable way to fix them using any sort of chemical.

Best course of action is to replace it with a new one. The cheapest crappy encoder you can buy on ebay (~1$) could work better than your worn-out encoder, at least for a while.

Of course that is not a suggestion for a long-term fix. If you care about the equipment, you could probably find a suitable replacement on any major component distributor (digikey for example, or RS components), for a couple of dollars.

Just for example, I just did a quick search on digikey trying to find something vaguely similar: BOURNS PEC11L Series - 11 mm Low Profile Encoder (datasheet).

As you can see from this datasheet excerpt (emphasis mine):

the expected minimum encoder life is 100k full rotations.
Assuming (optimistically) that the average life is twofold (200k rotations) and that the shaft is rotated on average 50 times a day (not uncommon in a control console in a work environment) you get 4000 days average life, i.e. about 11 years. Therefore what I initially said about your encoder being at its end of life, is perfectly reasonable.

All this assuming it is not some specialized high-reliability encoder.

If you want better advice post more information on that encoder (model number, better photos, info about the equipment, etc.).

Figure 1. Open the cleats.

I have successfully cleaned an encoder in an audio amplifier by squeezing the cleats on one side enough enable me to create a crack through which I squirted some switch / contact cleaner. (The cans with the straw attached help direct the spray.) Then the switch should be pressed together and the cleats spread once again.

The technique avoids the risk of parts flying off in various directions, not to mention the trouble of finding a suitable replacement, de-soldering, etc.

The best way to clean it is replace it. That encoder has a de facto-standard design, and replacements are cheap and readily available (I've got a box with dozens of them, from multiple manufacturers). While it is possible to disassemble and clean one, doing that without first desoldering it from the board would be tricky and could easily result in accidental damage to the board. (Basically, you'd use needle-nose pliers to squeeze the two split-and-flared flanges shown in the photo, as well as the ones on the other side, then lift off the top half from the blue housing.)

If you're reasonably adept with a soldering iron and with a desoldering braid and/or solder sucker, you should be able to replace it yourself without too much difficulty, though an iron under 40 watts or so may have trouble desoldering the anchors on the side. Otherwise, any electronic repair shop can do it for you easily.

I simply open it up completely (no desoldering is necessary), then with a small screwdriver scrape up any clean lubricating grease to save for reapplication after cleaning

Then use alcohol and brush to clean away any left over grease on the rotating disk and the small wipers, but be careful, the wipers are very fragile. Then take a pink eraser and clean the disk and tiny wiper pads, this will shine them right up, reclean with alcohol, then re-apply the saved grease.

This step is optional: very slightly bend the wiper contacts to give slightly more mechanical pressure. Reassemble and test. I have fixed countless mechanical encoders this way. Good luck.

P.S. This works on pots too, if the pot or encoder is very worn, you can carefully shift the wiper contacts left or right to contact a new section, this works wonders, almost new again.