Well :rolleyes: it's up to me to decide what "significant degradation" is when I use my lenses. Right? And to me any unnecessary degradation... and anything in the light path that increases the likelihood of flare etc.... is unacceptable.

Once I bought a really nice lens; the seller boasted that the thing had always been protected by a UV filter! Wonderful, right? Wrong. Turned out the damn filter -which had fingerprints on it- was stuck on there. I had to dremel it off, and it was basically impossible not to damage the lens threads while doing so (took much longer than an ordinary circumcision). Mind you, this is even more of an issue now with the plasticky lenses that easily crossthread. Buyers of "protective" filters beware: the solution can easily become the problem, and then what? Lenscaps are for when the lens isn't pointing at a subject; when the cap is off, just use a hood and be happy.

Anyway, back to glass cleaning theory...; ) surfactants micellize crud. That's why soap works in the shower. Water alone does not take off oil, and oil is what causes crud to stick so strongly that you cannot simply blow it off, so....

Accidental incursion of fluid into a lens is not an issue with MeOH and EtOH, assuming of course that it's done properly (by applying the solvent to a microfiber cloth, as people have suggested). These clean solvents have high vapour pressure and are gone almost instantly. They are readily available in purity levels high enough to leave no residue.