I've looked the 306 up on the internet, and so far I haven't found even one good review about it. But my mother kept it well cleaned and oiled (she used 3-in-one oil on it), and never had a bit of trouble with it.
From another angle, forget what you read or haven't read on the 'net. You have first hand knowledge that your mother used the machine, and in spite of abusing it with the wrong oil, it gave her no trouble. That
should answer your machine question right there.
The only thing that might be a cause for concern is the machine takes the 206-13 needles and they are a bit difficult to find. Especially in the larger sizes.
From the memories of the abusive behavior angle, why not take the machine, clean it up. Bring it back to life and put a positive aura to it? A sewing machine is after all only a mechanical tool regardless of how we goofy humans try to humanize them.
Then after making something happy with it rehome it to someone who can use it if you need to.
JMNSHO
Joe