Both Mobil 1 and Mobil Delvac 1 are true synthetics, not blends. Yes, I do agree that sythetics can extend drain intervals, but you should have an oil analysis test performed to ensure that the oil is not degrading or accumulating harmful particles (dirt, metal, etc...). Many labs perform this oil analysis such as Mobil, Cat, Detroit Diesel, and independants such as Blackstone Labs, but these tests do cost money. The $20 fee that Blackstone Labs charges for analysis will just about fill up my engine, even at $5 / quart. I'd rather just change the synthetic oil at the recommended interval and enjoy the benefits of cooler engine temps and longer engine life. Here is a quote from Mobil's website stating the other benfits of Delvac 1: "The Synthetic Alternative Mobil Delvac 1, a synthetic engine oil, measurably extends the service intervals and engine life of high output diesel engines running under heavy loads over long distances. In comparison to high quality mineral-based engine oils, the synthetic technology of Mobil Delvac 1 handles greater quantities of soot and other combustion by-products, while providing outstanding resistance to oxidation and sludge formation, thus keeping engines cleaner. Mobil Delvac 1 is an especially advantageous choice for vehicles routinely used in extreme temperatures. Engine start-up is quicker and easier on electrical systems when it is cold and when it is hot, all critical engine components receive effective lubrication. Mobil Delvac 1 also offers excellent resistance to viscosity increase and the potential for improved fuel economy compared to conventional viscosity grade products." I am using Bobcat brand oil filters that I get from my dealer for ~$5 each. This is not a bad price. Are there any other brands that people are using?