My machine is squeaking as I sew (had it serviced only a month ago). How do I oil it?

Perhaps you should take it back where it was serviced. The machine should have been oiled when it was serviced. There could be something else hanging up that should be addressed.

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The instruction book should have oiling instructions and helpful diagrams and an oilcan should be one of the accessories supplied with the machine if oiling is required.
Plastics have replaced metal in many ways and removed much of the need for oiling. Unfortunately plastic often squeaks louder than metal. Some plastic tolerates oiling very well but causes deterioration of other plastics and a silicone grease or spray should be used.