This is an article I drafted up. I am open to additions and modifications! Not meant to stir up brand wars, but instead inform the consumer to make an educated decision! https://sites.google.com/site/aox4f250/home/writeups/whatoilshouldiuse What Oil Should I Use Well, you have just asked a very complicated question. Truth be told there is no right answer. For the purposes of this article, I will be focusing on diesel oil, commonly used for the 7.3L and 6.0L. There are tons of options available, from a variety of suppliers. I will not tell you which oil to use, but instead give you the information you need to make an educated decision. API Ratings The first and most important think you need to know is oil ratings. The 7.3L requires a CH-4 rated oil (or better), and the 6.0L requires a CI-4 rated oil, (or better). CH-4 was introduced in 1995, which was followed up by CI-4 in 2002. CI-4 was introduced to meet new EPA standards set for 2004. The current oil rating set by API is CJ-4, which was created to meet the needs of new EPA standards set in 2007. CI-4 and CJ-4 oils are backwards compatible, so you may use a CI-4 or CJ-4 oil in either a 7.3L or 6.0L. Ultimately, as long as the oil you choose meets the required API rating, you will probably be okay, although you might also want to look into the right oil weight for your needs. Oil Weight OEM specification from Ford is a 15W-40 weight oil. However Ford does suggest a lower weight oil for cold weather operation, and many people choose to go this route as it helps with cold starts. 0W-40, 5W-40, 10W-40, and 15W-40 are all acceptable weights. The lower the weight, the easier it will be for your engine to pump at cold temperatures. Beyond rating and weight it turns into a lot of preference. Popular brands include Shell Rotella, DELO, Valvoline, and AMSOIL. Most brands have both synthetic and conventional oils. Can I run Synthetic Oil? Some people believe that they cannot run synthetic oil, or that once they use synthetic oil they cannot go back to conventional oil. These are myths. Conventional and synthetic oil are interchangeable. You dont need to flush to switch back and forth, and you can switch as often as you want. Synthetic oil is more expensive, so some people only run it during the winter months to help with cold starts, then go back to conventional oil during warmer months to save money. Other than synthetic oil generally being lighter weight oil, there are a few advantages. Synthetic oil generally has more advanced additive packages that clean and protect your engine. Every brand, and sometimes lines of oil within a brand, have unique additive recipes. So Which Brand? This is up to you. Once you meet the requirements of weight and rating, you need to decide what is best for you. Some people like one brand over another, some people go with what ever is cheapest. If you are so inclined, you can research the various additives each oil has by searching for Virgin Oil Analysis' (VOA) of the various brands. Also, some brands seem to hold up better than others. This can be seen by looking at Used Oil Analysis' (UOA). Popular Oils include Synthetic: Shell Rotella T6, Valvoline Extreme Blue, Mobil 1, and AMSOIL Conventional: Motorcraft, Shell Rotella T, Valvoline Premium Blue, Mobil 1, DELO Oil Base Stock All motor oil comes from what is called a base stock. The API defines these stocks into 5 groups Group I and II: these are mineral oils derived from crude oil Group III: this is a highly refined mineral oil made through a process called hydrocracking. Group IV: these are true synthetic oils, known as Polyalphaolefin (PAO). Group V: these are synthetic stocks other than PAO's and include esters and other compounds. Many Group III oils are marketed as "Synthetic Oil" because of the amount of processing they go through.