I've used the dilute mayo for years, Patty. No problems so far. As you say, it's cut pretty well with water, and I do also wipe most of it off again once I've got the hard water deposits off. I can see how using straight mayo might be too much, but I'm guessing it also depends upon how fine the oil is - we do use oil sprays on plants to battle scale insects, too. As I recall one of the products is called "ultrafine" oil. I also spray a dormant oil spray on the orchids in winter, so it can't be all that bad for them.
Got this as a tip from someone who shows her orchids at big shows up north, and she's used it for many, many years.
