docswitzer wrote:I've been experimenting with staying around 68-74 mph and have the mileage up to 35mpg. Coming up on 8k miles. One thing that puzzles me is my dealer gives 2-free oil changes, but they use conventional 5w-30. Has anyone noticed a difference with synthetic or synthetic blends?
The greatest importance of synthetic oil is that it increases the chemical stability and reduces volatility of the hydrocarbons in your oil (most often by adding a double bond at one end). As lighter/thinner hydrocarbons tend to be more volatile, synthetics allow you to use a thinner grade of oil. Just like running through air versus running through water, a lower viscosity oil will improve fuel efficiency. And it means that your oil will not thicken as much over time from lighter hydrocarbons burning off.
If you switch from a GF-5 rated conventional oil to a full synthetic of the same grade, the difference will be negligible when new and a fraction of a percent improvement at the end of the oil's life. The fuel saved by this tends to be less than the cost differential, but it is a measurable improvement in fuel economy(unlike HHO, CAI, fuel line magnets, vortex generators, etc.).
Switching to 0W20 full synthetic will make a measurable difference of a percent or two that justifies the increased oil cost, but your engine needs to be able to handle it. Typically, thinner oil requires smoother engine finishes, tighter tolerances, special coatings, etc. If you have a new Altima, Sentra, or Rogue that was designed for 0W20, then you should use it. Nissan recommends 5W30 HR16DE . You may be able to get away with 0W20, as it will still improve fuel economy in engines not designed around it. However, if you are going to try something outside of the manufacturer's recommendation, you should be careful, check your oil levels regularly, do oil analysis to measure wear, listen for valve clatter, etc... may not be worth the risk/work.
http://www.swri.org/3pubs/brochure/ae/p ... nceVID.pdf