Ok guys. Quick question. Was Castrol Syntec ever a "True" Synthetic oil? Reason I ask is, at a walmart store, they have about 12 bottles of the older syntec 10w30 I believe. It is the older square bottles and the ratings on it were SJ. If they use to make a true synthetic is would this be it? if it is would you buy it? One more quest. I like to run thicker oils within reason. Meaning, I wouldn't run a Dino 20w50, but I would run a Synthetic 15w50. Because the flow rate on that Mobil 1 15w50 is as good as most 5w30 Dino oils. I know my truck calls for 5 or 10 w30 but I would assume they didn't have synthetic on their mind. I do not burn or lose any oil that I notice. But Ido have 113,000 miles on my truck. I have read some people mix like a 15w50 with 5w30. What would I have to mix for a 4 qt system to make a Mobil 1 10w40. That's about where I would like to be somewhere in that range. Thanks guys.