Here is my understanding of the situation.
A. METAL
Machines with metal gears need oiling to reduce friction and, therefore, wear on those metal parts. Where metal moves against metal, oil it.
B. PLASTIC
Newer machines tend to have plastic gears, which I believe are generally made of nylon.
i. Plastic, unlubricated
Petroleum-based lubricants should not be used on plastics as plastics are weakened by them. Other lubricants such as lithium- or silicone-based ones. Hence the warning not to oil your new machine.
i. Plastic, self-lubricating
It is possible to lace some plastics with materials such as PTFE (aka Teflon), silicone, or graphite. These greatly reduce friction on the gears' surface, and the plastics are known as 'self-lubricating'
It is highly likely that if your machine has plastic gears, it is using self-lubricating nylon ones.