Castrol Syntec is NOT a full synthetic no matter how they choose to advertise and decieve us. Actually, it might be depending on what definition you use for the word "synthetic." Castrol Syntec was reformulated in late 98. Before Syntec was reformulated, it was actually a full synthetic using a full synthetic base stock with PAO's just like Mobil 1 and 90% of the synthetics out there. Once Syntec was reformulated in late 98, it was known that Syntec did not contain any PAO's. In fact, Syntec uses a Group III base stock which is a hydrocracked highly refined mineral oil with some synthetic additives. This is no secret, because Mobil immediately sued Castrol in court over whether Castrol could rightfully call this new reformulation of Syntec, a full synthetic as they still advertise. If the base stock was a highly refined Group III base oil, then how could Castrol rightfully and legally call it a full synthetic? Unfortunately, Mobil lost the lawsuit because of how the term "full synthetic" is defined by the API or SAE(can't remember which one). A full synthetic motor oil is defined on it's ability to perform, not by what it's made or composed of. The scam here, is that if a company takes a cheapa$$ dino oil, hydrocrack it, add a few synthetic additives, and it ends up meeting the minimum specifications of how a synthetic oil is supposed to perform, then the manufacturer is allowed by this vague and deceptive definition to sell and market thier oil as a "full synthetic!!!" If you guys like Castrol Syntec, that's fine because it's a good oil. But don't expect it to be a full synthetic oil composed of Diethers(Redline), or PAO's(Mobil 1, Amsoil, etc....)because it's not. Castrol Syntec sells for $4 a quart around here, therefore if you would like an oil that is marketed as a full synthetic, yet is not a full synthetic, then you have many choices that cost a lot less. There was a controversy on thsi topic over on LS1 about 2-3 years ago over the Syntec issue, and Castrol not only threatened a lawsuit against a couple individuals not to mention Amsoil as well, but they also forced them to remove those articles that "exposed" the truth about Castrol Syntec. ...This is from a Ford truck message board I used to do a little browsing on.