Hello, I have what I consider to be probably a dumb qustion but here goes. I recently purch an rx7 1989 convertible with the non turbo engine. while scanning the specs in the manual I noticed that the engine oil section called for a certain type and number followed by the words mineral oil. what is up with this. doesn't the rotary eng use reg oil. If its a dumb question I apologize but I found it strange.