How to clear off oils in a way that will not damage the plastic? Especially with the pneumatic cylinders, so it has to not make the metal rust too. Also, it is oxidized vegetable that is stuck in there, inside the pneumatic cylinders. And it started to make it all sticky inside. I used some Rem oil, which is a gun-cleaning grease that has Teflon in it, and it didn't really help. It just stayed the same.
Your question is general, so I'll try to answer generally, but I'll use the pneumatic cylinder as an example since you mentioned it.

If applicable, the first thing you'll want to do is break down the part(s) as much as possible. Open up the pneumatic cylinder and separate out the parts:



I'd recommend cleaning everything separately, lubricating the necessary parts, then putting everything back together.

For cleaning off oil and oil residue, you'll want some sort of solvent that is safe to use on ABS, such as WD-40. That should break down more viscous oils, gunk, and rust, but will also remove printing from printed parts. You can scrub ABS with a soft cloth without causing damage. If that isn't enough, you can get more aggressive and use something like steel wool or sandpaper, but this will cause light scratching. I've successfully use steel wool on the metal portion of pneumatic cylinders to remove rust, and it works nicely.

Once you have all the heavier oil and grime washed away, you'll want to use a common dish washing detergent to remove the remaining oil. I just put a few drops with water in a jar and shake with the parts inside. Rinse everything off, and repeat as needed if there is still any oil left on the parts.

In order to prevent rusting and ensure proper functioning of the parts, once the parts are clean and dry you'll need to now lubricate anything that was lubricated before since we've removed all the oils and lubricants. You can use any oil that is safe on ABS for this. I've use Vaseline on the rubber piece and cylinder shaft before and it has worked well.