After some time. It will be your brass nut disintegrating.
That makes sense.
However, if it becomes yellow of brass particles, there is a huge amount of wear going on, and you definitely need to check the wear on your nuts.
You should really not use (regular) WD-40 for lubrication, use a lubricant such as 3-in-1 oil for axis lubrication and grease for the lead screws.
Granted there is not much load on a 3D-printer rods and screws so you should still be ok, (regular) WD-40 is not a pure lubricant, nor useful for long-term lubrication, it contains solvents and other chemicals used for water displacement and cleaning.
WD-40 is a brand name.
Bambu Lab itselfs recommends WD-40 Multiuse product to lubricate printer axis.
I saw, and its a bad suggestion because most people are just going to buy regular WD-40, or already have a can of regular at home and miss the distinction or just use it anyways.
Also kleenex, Q-tips and vaseline are band names, but they have become the meaning of certains types of products. Everyone thinks WD-40 is just a lubricant when only a specific type of WD-40 is even suited for lubrication.
Eh? Can you link me to the two different products? As far as I am aware, they are exactly the same thing.
WD40 multiuse is just the WD40 that people refer to?
They likely refer to the plethora of other products the brand WD-40 sells, like industrial degreaser which would be very bad to try and lubricate your printer axis with.
Note, this is from my knowledge of WD-40, if Bambu recommends something I am not going to challenge it, you should always follow the manufacturers recommendation. But I would personally never use anything but lubricating oil for the rod and bearing type of moving parts in a 3D printer. (and grease for the lead screws).
Still, “regular” WD-40 is not suited for long-term lubrication as it doesnt contain enough lubrication oil, and also contains other chemicals for its penetrating, rust dissolving and demoisturizing capabilities. It can dry out rubber over time, so the drive belts for the x-y axis of a 3D-printer could possibly be damaged by it if someone would not be careful enough when spraying it on the axis.
I’m surprised Bambu recommend it at all. Lubrication oil like “sewing machine oil” or lubricating mineral oil is far better at lubrication of this type.
Perhaps its part of the reason some can get stripped lead screws? They sprayed WD-40 everywhere and as a result dried out the grease on the lead screws?
When I say I had drink a coke I mean a standard coca cola, not a sprite or a Fanta, those are coca cola brand as well 
Dudes I had read the bambu Lab papyrs about lubricants, they wrote WD-40 and be sure it’s the classic one.
WD-40 is the exact name of their main product, I have cans of that all around all homes, legit cans and those tiny they give for free during average Motorsport event.
I use it on everything and I use it as a cleaner as well, on metal and plastics. Engine block become shiny and nice when you rub WD-40 on it
I whole heartedly agree, I wouldn’t consider using WD-40 anywhere near my printer. I guess my point was more that Bambu are literally referring to standard WD-40, not a “multi-use” special product. It’s just bog standard WD-40.
Average people lecturing of these didn’t read instructions. I really had read all manuals. That’s why I am using food grease instead of heavy industries grease for Caterpillar’s axles