Largent80 wrote:Scott, which synthetic oil do you think is best?
Mobil 1 is by far the best-formulated synthetic passenger car motor oil for gasoline engines available on planet Earth. It is factory-fill for the Corvette and has been so for the last 15 years. Mobil 1 formulates their product to exceed the performance standards of every engine oil on the market, including those found in the racing market, like Red-Line, Torco, or Royal Purple (which I wouldn't put in my worst enemy's lawn mower). A close second would be the Pennzoil Ultra product line.
When it comes to diesel engines and, more-specifically, the new clean-burning late-models that use advanced EGR systems, the Shell Rotella T6 synthetic 5w40 is hands-down the best. The technology employed in the additive package alone cost them several million dollars to develop. No engine oil has been tested as extensively as the Rotella product line, and there is simply nothing that comes close to the level of protection it provides.
The thing about synthetic base oils that most people are unaware of, is that today's engine oils marketed as synthetic contain no true synthetic oil, or poly alpha olefins at all. Rather, highly-refined petroleum group III base stocks are put through a hydro-treating process using a catalyst which creates a base stock which duplicates the performance properties of a true synthetic oil. These newer base stocks can be called synthetic because Castrol won a court battle arguing that since this refinement process is technically a "synthesis", and the resulting product does the same thing, it can marketed as synthetic. Since PAO synthetics must be blended with esters to prevent attacking the seals in an engine, the cost to blend a true synthetic exceeds that of using highly-refined group III's. So, oil manufacturers have switched base stocks to the group III and group IV in order to make the bottom line work.
Mobil 1 is the only oil formulation I am aware of which still uses a proprietary tri-synthetic formulation that includes a true Poly Alpha Olefin component.
Another thing, for those that think that a synthetic oil protects an engine longer than a conventional oil, you are very, very wrong. Sorry. Regardless of what oil is used in an engine, the exact same combustion by-products are produced by that engine and are entrained in that oil in the same way. Un-burnt fuel, moisture, soot, dirt (silicates), etc. find their way into engine oil as the engine runs, that is what makes it get dirty. Additives in oil contain detergents and dispersants which surround the molecules of contamination and hold them in suspension until the oil is drained. Because these additives do not replenish themselves they are sacrificial to the process. Thus, there is a limit to how much contamination can be collected and kept from forming a deposit. All engine oils, synthetic or conventional, contain the same type of detergents and dispersants and they perform the same regardless of the base stock they are blended with. If you extend your oil drain interval, you are subjecting the inside of your engine to deposits like sludge and varnish which encapsulate water leading to corrosion and formation of acids.
Engine oil should be changed at a point in its life during which time all of the additives in the formulation are still doing their jobs. Additionally, laboratory testing has shown that the average engine holds 10-15% of the dirty oil inside during an oil change. This means that the moment you start your engine after the oil change, your fresh oil is being contaminated by what you just drained out. Laboratory testing has also shown that the point at which the additive packages start to lose the battle is 4500 miles. Me? I change my oil at 3500 miles. A good balance of cost vs. protection. This holds true across the board, gas or diesel.
Sorry for the novel, I'm passionate.