How can you get streak-free, crystal clear windows? The answer is treat the process like making meringue. Yes, you heard me. Cleaning windows is like making meringue. People who make a lot of meringue know that the key is to keep all of their equipment as clean as possible. If the beaters or bowl have even traces of oil or grease on them, the meringue will not reach its ultimate level of fluffiness.
Some extra-exacting bakers will wash the outside of the eggs and wear gloves so not even a hint of oil reaches those precious egg whites.
While you don't expect your windows to be fluffy, you do want them to be streak free. Repeat after me: use paper towel. I know, I know I am breaking my stay green rule. I suppose my only reply is that sometimes we break rules to attain an optimal experience. Cloths you use around your house are bound to get oil on them. Oil is notoriously difficult to clean off and out of material so no matter how many times you wash those cloths, no matter how hot the wash water is, over time they will build up a bit of oil residue. Such is life.
If you want streak-free, crystal clear windows, use paper towel.