Having been off all direct fish oil supplementation for over a month now, I thought it interesting that my shoulder issue has improved greatly over the same time period. It's quite possible that the effects of fish oil supps are not immediately apparent (in the case of joint health) but that there is a positive long-term effect that continues on for a time after supplementation ceases. (Subjective I know, but I have noticed that the skin on my elbows seems to be a little drier again, although not as bad as it was before I started supplementing.) Of course, I'm still eating some oily fish every week along with plenty of virgin olive oil, so that will count for something (perhaps quite a lot).
I do intend to start up fish oil supplementation again soon but I will return to 10g a day rather than a higher dosage and see if my shoulder remains healthy. I know there are lots of studies showing the benefits of fish oil in the diet but I don't see the point in taking a high dosage of the stuff (30g+/day) unless it is fully evident that there are extra benefits over and above a regular lower dosage along with good oily fish and other good omega-3 rich oils in the diet.
This is the sort of article out there on the interweb that discusses supposed benefits of fish oil supplementation:
Human study: omega-3 fatty acids delay molecular ageing
Any studies sited usually have subjects taking relatively low levels of fish oil supplementation compared to the levels prescribed by some in the weightlifting community - often < 10g/day.