Hi, My Canadian Oxford Paperback Dictionary lists 'plus' as a preposition but not as a conjuction. It also says 'The use of plus as a conjunction is considered incorrect by some. ' I heartily agree.
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In many of the uses of plus as a preposition, you can substitute "with" or "also" for "plus". The example of plus as conjunction is a construction where one could substitute "and" for "plus", although for least one of the preposition examples "and" could also be substituted. However, the example of "plus" as conjunction shows it being used to make a compound sentence.