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From all that it appears that the isopropanol is used to remove the greasy marks and the water based cleaner, like diluted Windex, removes the water soluble ones.
As for how often to clean, well, I look at my eyepieces and remove dust with the blower brush then check for any smudges. If they are obvious in good light then its time to remove them. Most times it only takes the removal of dust to keep them clean unless you have the unfortunate habit of pointing out the things of interest in your eyepiece with your finger!!
Cheers
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