So a CI-4+ can be synthetic or a mineral oil, correct? We are using Service Pro Heavy Duty CI-4+ 15W-40 and in the catalog we order from, they have a heading for synthetic and semi-synthetic and the CI-4+ is not listed under either catagory. Also, looking at product info I notice that the Service Pro we're using has a much higher pour point than other oils that are specifically listed as synthetic. Usually as much as 20 or 30 deg. higher. I'm using some Auto-Rx in my truck and am hoping to use the oil we have on hand but I know they advise against using synthetics with the A-Rx.