NOTE
the pins on the back of the eye can corrode if they are left in a state with paint on them (Take off the eye when cleaning, and dry the tracks on the back out properly) and cause eye misreading problems. If this has already happened, scrib around the tracks with a pointy thing (needle, scrib etc) to make sure the pins are not shorting out.
The later eyes apparently have a better coating to reduce this effect but as you all know, the paint is very corrosive (if you have ever seen a car that has been - accidently - shot you will knwo what I am on about) so they can 'fur' up over time.
I have just swapped mine out, but mine has been going strong since Oct 2002 so I am not complaining.