+1 for diluted IPA and a microfiber cloth...a couple of my past (and present!) SCTs were bought used and being a bit OCD I cleaned the corrector plates, one of which was quite greasy
I use the Baader Wonder fluid + cloth, but the fluid is just diluted IPA (isopropyl alcohol not a pale ale
) if the prints were fresh on a pristine corrector they should lift pretty easily but don't expect a streak free finish without a lot of dedication...one of mine took several sessions
but neither the prints or any streaks won't be visible through the eyepiece.