Thanks again, Roland. Your 55mm f/3.5 is also of the latest series, so agree that it is likely similar to my copies. It would be interesting with an infinity test too. I agree that oily residues could have been an undetected issue on the f/2.8 version I tested. No filter was attached, of course. My coworker still has his lens; he had it serviced in the mean time and will bring it in to work later in the week so that I can get to examine it again.
I had the 55mm f/2.8 for inspection again today, and opposed to the saying above I am not sure it had been serviced; the helicoids were still very stiff, and the aperture started acting up towards the end outside at about 0°C. However no oil was visible on either side of a fully closed diaphragm. Serial no is in the first series, 433xxx.
This time it was hard to see much difference in contrast; if any it is very subtle. It is possible that the conditions contributed, but it is also possible that the lens was cleaner one way or the other than last time. What was consistent with last test was that the 55mm f/2.8 was leaning towards being warm/green vs. colder/blue-magenta for the 55mm f/3.5. Due to trouble with consistent focus (too bright to use live view for accurate focus) it was hard to detect any difference in resolution near infinity.
100% crop 55mm f/3.5 AI at f/8
100% crop 55mm f/2.8 AI at f/8
The lens looked reasonably clear, although with a little blue shimmer internally that I could not quite figure out.