I am a retired (No longer actively practicing) Nurse Practitioner. So this post is from experience, it is NOT medical advice. I worked with an orthopedic hand surgeon for about 10 years both in surgery and in the office. We treated many hand/upper arm injuries from hydraulic fluid being injected into the hands and arms. In every case where there was any amount of hydraulic fluid injected, it was a medical emergency and we took all to surgery to open the wound, flush it thoroughly with a pulsa-vac (kind of a medical pressure wash system) to clean out all the hydraulic fluid. We saw a number of post injury patients who had not been treated that way. Some had just been stitched up and sent home to heal. Those who had no treatment had tendon adhesions, significant scarring, rigidity of fingers and wrists and none had full function. For anyone who has hydraulic fluid injected into the skin, I urge an immediate visit to the ER and an immediate consult with an internist if the injection is on the main trunk or with an orthopedist if the injection is near a joint or on the hands.
Now, with the original poster, it's been three years since the injury, so anything that's currently going on is most likely NOT a result of anything actively damaging your body, rather at this point, I'd think you're dealing with the after effects of the injury. IBS can be caused by a number of other things (as well as by the hydraulic fluid). It can be something as simple as drinking dairy products if you're lactose intolerant. Some of us, as we grow older stop producing Lactase, the enzyme required to digest dairy products. It also could be caused by scarring in the small or large intestine caused by the hydraulic fluid. But in any case, it's not actively damaging tissue after 3 years. The damage is already done, now you need someone to find the damage and treat it properly. It may well be something other than the hydraulic fluid. As we age, things happen.... I'm 66 and can honestly say I can't do things as well as I 'used to"
I'd suggest you find a competent internist or Gastrenterologist and have a complete workup for the current symptoms. Be sure to stress that you had the hydraulic fluid ingestion about 3 years ago so they are aware of the accident, but they need to focus on the current symptoms (with past history) and treat objectively what they find. They really don't need to focus on trying to "prove it was the hydraulic fluid accident" rather, they need to find the "current" condition and treat from here forward.