I have a 2002 Civic EX, automatic with ~79k miles. I've changed the oil about every 3k miles wherever I could get the best deal. I'd also buy the cheapest gas, but I would regularly use gas treatment (STP mostly). Last year, I bought a house so now I do the oil changes myself in the garage. I'd always used Fram filters and cheap Wal-Mart SuperTech oil, 5W20 grade dino. I started tracking my mileage about when gas prices were close to $3/gal in early May. I also switched to using Chevron gasoline. At the time, I was doing a lot of highway driving, and I was getting around 33-34 mpg with the A/C on about half of the time. My driving pattern changed after a couple tanks, and I was now getting about 29-30 mpg for short 10-15 miles trips, 3/4 highway, 1/4 city driving with the A/C on most of the time. I had a set of fairly new Bosch Platinum Splitfire plugs in for this period until just recently. My second to last oil change was Castrol GTX 5W20 with an OEM filter (I was smartening up a bit after reading forums like this). About two week ago, I did a bunch of stuff to my car because I was unhappy with my mileage. First, I changed out the Castol GTX, flushed the engine with some fresh left over oil that I had laying around, and dumped in some M1 5W20 along with a can of Lubro Moly MoS2. I started it up, and it almost instantly felt smoother that ever before. My model of Civic is known for pathetic noise insulation, so I could easily hear the difference. It was especially noticeable when accelerating in first and second gear- smooth as butter. (Aside- I didn't like how the transmission was feeling at the time, so I drained and filled with Honda ATF, which made it a little snappier between gears.) I also changed out the Bosch plugs with a set of OEM plugs (very expensive!). I did not notice a diffence in how the engine ran with the new plugs, and the old ones didn't look too bad (only 7k miles on them). The last thing I did was to fill up the tank at a different station (Valero) in the morning (b/c I read that gas expands with temperature, and it's been over 100 a lot lately). It's also the cheapest gas around. That tank gave me almost 36 mpg, for a solid 20% mileage improvement over a very consistent 29-30 mpg for 3 months. I had the A/C on most of the time and my driving habits haven't changed at all. Does anyone care to speculate what change(s)contributed the most to my mpg increase? I feel that it's mostly due to the moly.