What is the difference between notion and conception?
According to The Free Online Dictionary, here is a comparison of synonyms of idea:
Synonyms: idea, thought, notion, concept, conception
These nouns refer to what is formed or represented in the mind as the product of mental activity.
Idea has the widest range: "Human history is in essence a history of ideas" (H.G. Wells).
Thought is distinctively intellectual and stresses contemplation and reasoning: "Language is the dress of thought" (Samuel Johnson).
Notion often refers to a vague, general, or even fanciful idea: "She certainly has some notion of drawing" (Rudyard Kipling).
Concept and conception are applied to mental formulations on a broad scale: You seem to have absolutely no concept of time. "Every succeeding scientific discovery makes greater nonsense of old-time conceptions of sovereignty" (Anthony Eden).
One can have many notions about a single concept but a notion usually cannot be about many concepts.
According to dictionary.com, a notion is a vague or imperfect conception.
Thus we may talk about a person having "odd notions" concerning something. On the other hand we may a person has a very "clear conception" of a topic.
So there is a good deal of overlap in meaning but if you wish to emphasise the imperfect nature of an idea you may use notion.