George,
I've had good luck with several (>10) Retina's using Kodak Lens cleaning fluid and lens cleaning paper wrapped around a q-tip. That is for all surfaces EXCEPT the semi-silvered beamsplitter mirror. For that, and only if it is really bad, I breathe gently on it and very gently wipe it with the tissue on the Q-tip. I've cleaned the 1a, II, IIa, IIc, 2-IIIc's, 4-IIIS's, and 2-Auto 3's. All came out clear. You have to do some disassembly to get to all the surfaces. The inner eyepiece comes out with a retainer of the IIIS' and some of the others. The IIc and IIIc were the worst, had to take out the front objective and the frameline mask.