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When cleaning plants, skip the mayo, milk and conditioner!

It doesn't help, and might hurt.

There is an awful lot of advice available on the internet, if you hadn't noticed. Most of it bad. A lot of it sounds good, or is the kind of thing that sounds so unlikely that it MUST be right. Especially if someone asserts that their dear sainted great grandmother used to do it that way. 

Look, our great grandparents used to do a lot of things. Drive around drunk without seat belts. Die of polio. Douche with Lysol. I'm just saying, it's not a guarantee that they knew "the right way to do it" back in the olden days.
I feel like the advice to clean your house plants with milk or mayonnaise or (heaven help us) hair conditioner falls into this category.

First of all, these substances do not help make your plant cleaner. The only thing that cleans your house plant is, well, cleaning it. If you wipe the leaf with a damp cloth, that is as clean as it's going to get. (And your plant will thank you for it!)
What these products do is add shine. "Shine" is not something that the plant itself cares about. The relative shininess of a leaf is strictly a cosmetic issue for us humans. If you asked the plant, it would probably tell you that it thought its leaves looked just fine, thank you.
There are now (and have been in the past) many products that purport to make your plant's leaves shinier. Most of these are one form of oil or another. The oil, like furniture polish, does indeed make the leaf shinier. It also blocks the leaf's pores, which makes it difficult for the plant to breathe.
The worst of these oils, like the leaf shine products of yore, actually make the leaf sticker at the molecular level. Thus making the leaf more likely to attract dust: a vicious circle. I suspect that this would be the case for mayonnaise and conditioner, and possibly for the milk concoctions as well.
The only exception to the "shinier leaves are not better leaves" is the case where hard water stains have developed on leaves. You often see this in plants from a greenhouse with overhead watering. The hard water residue can give the leaves a powdery or dull look.
To solve this problem, simply mist the leaves with a weak vinegar solution (1 part vinegar to 5 parts water will do). Wipe the solution away, and the hard water stains should come with it.