Back in the 1950's the military developed WD-40 as a rust preventative solvent and de-greaser to protect missile parts. It quickly became popular in households everywhere when people discovered other uses for the product such as a cleaner, rust-prevention agent, squeak-stopper and more. It also works wonders as a light lubricant on small items like hinges, locks, and toys.
Sewing machines on the other hand are far too heavy duty and fast moving for the lubricating power of WD-40 to have any affect at all. As a matter of fact, WD-40 will actually strip away any existing lubricant and leave your drive train dry - metal on metal. Eek! Basically, spraying this stuff into your sewing machine is worse than using no lubricant at all! Don't do it.
What kind of oil should you use to lubricate a sewing machine? ONLY sewing machine oil. Not 3 in 1 oil, not WD-40, not engine oil, olive oil, or fax machine oil.
When it it okay to use WD-40 on your sewing machines? Many of you already know I have an affinity of antique and vintage sewing machines. I admit to using WD-40 on these, but not as a lubricant. I only use it with my sewing machines as a solvent. It does a great job breaking down OLD gooey gunky grease and oil check out She's a Sewing Machine Mechanic for detailed instructions on this method.
Never ever ever use WD-40 on long arm quilting machines or industrial sewing machines as these types of machines because they may have porous bearings.
BERNINA model sewing machines require a special high performance type sewing oil which can be purchased through your local dealer.