The use of wd-40 has been debated for a long tom in the yoyo community.
WD40 does not contain water. The w in the name does stand for water though so that is where the confusion usually happens. Wd-40 actually stands for “water displacement 40th attempt”. It took 40 tries to get the formula right.
It was originally designed to be sprayed on metal surfaces to remove water and then leave a barrier of oil to keep water out thus preventing rust.
The problem with using it as a lube for yoyo bearings is in the solvent that the oil is mixed with. It is basically mineral spirits. It helps get the oils where they need to be then when it evaporates it leaves behind the oil to do the protecting. This mixture just does not work well with bearing for some reason. After the solvent evaporate the oil that is left is gunky and slows a bearing down pretty bad. I have read on the forums over the past 8 years ago about many people trying and not one of them were happy with the result. I have also heard stories about ruined bearings. I would think that they would not be ruined just gunked up and a good cleaning would bring them back but I have never tried.
I would suggest you take the advice from everyone that has been a round for a wile and skip it and get yourself a decent thin oil. One of my favorites is “Break Free” you will find it in gun shops or sporting shops like “Dicks” that carry gun cleaning accessories. It is very thin so the bearing spins real nice yet it provides a good amount of protection from rust.
Sorry for the wall of text. Hope it helps though.
I just looked up the details on what is in wd-40. At some point they started using carbon dioxide as the propelent. What makes it spray. C02 is an acidic oxide. Yes it evaporates pretty quickly so I dont know how much damage it could do to the bearing but for this reason alone I would stay away. Just found it very interesting and wanted to share with all you guys.