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I know Pennzoil used to be paraffin based and that will lead to a gummed up engine, at least in a gasser. I am not sure if the oil for diesels is different but I don’t use Pennzoil in anything I own
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There are two basic types of crude oil, naphthenic and paraffinic. Most conventional engine lubricating oils today are made from paraffinic crude oil. Paraffinic crude oil is recognized for its ability to resist thinning and thickening with temperature, as well as its lubricating properties and resistance to oxidation (sludge forming tendencies). In the refining process, the paraffinic crude oil is broken down into many different products. One of the products is wax, and others are gasoline, kerosene, lubricating oils, asphalt, etc. Virtually every oil marketer uses paraffinic base stocks in blending its engine oil products.
Many people believe the term paraffinic to be synonymous with wax. Some have the misconception that paraffinic oils will coat the engine with a wax film that can result in engine deposits. This is not true. The confusion exists because paraffinic molecules can form wax crystals at low temperatures. In lubricating oils, this wax is removed in a refining process called dewaxing."