There is a great deal of confusion to whether, where, what with, and how much to oil Bernina machines. Your owner’s manual is indispensable in this regard. It not only tells you where and how much to oil, but also which oil to use.
First, a word about the oil itself. The safest oil to use is that which came with the machine, or that which can be purchased from a dealer. Bernina oil should say "Bernina" on the tube. If you have any doubts, for instance if you have purchased a used machine and the oil is suspect, it’s probably a good idea to throw that oil out and get some more. Bernina oil is very light in viscosity, and is almost clear.
The area that is most critical to oil is between the shuttle and the shuttle race. (See below) Oil this about every second time the machine is used for any length of time. Just a half drop will do.
Over-oiling just makes a mess. It’s also a good idea to oil the pin on the shuttle that the bobbin case fits over, and a little on the bobbin case latch.
Having said the above about the type of oil, I will say that I personally have used Singer brand oil (found at any Wal-mart) with good results. "Three-in-One" oil or any kind of motor or vegetable oil should NOT be used on sewing machines at all. I have seen machines virtually ruined with the wrong kind of oil.