I drive a 2000 Cadillac Deville. I have been using Wal-Mart Super Tech Synthetic oil 5w or 10w 30 for over 2 years. The car burns oil (by design) so I want to go to a less expensive oil Super Tech conventional 5w or 10w 30. I heard through the grape vine that mixing synthetic oil with conventional oil you will get a "foaming effect" of the oils mixing? 1. Is this true? and 2. Will it harm the engine?