The frequency with which you change your oil and filter are more important than the type of oil you use. That being said, I use Mobil 1 10w-30 and a Napa filter, and change them both every 3000 miles. I also add a quart of Slick 50 every 50k miles, too, but that's just me. Does synthetic make a difference? I think it does.
When I bought my truck, in August 1996, it had 19k miles on it. The first thing I did was to change the oil and filter, and I installed Valvoline 10w-30. Over the next 6 months, I added several things, including mechanical AutoMeter Sport Comp oil pressure, water temp, volt and vacuum gauges. I also tuned it, and changed the oil at least one more time, maybe two. In July 1997, my g/f and the kids and I made a 14 hour drive up to Oregon. I decided a few weeks before the trip to change the oil to synthetic. I installed Mobil 1 10w-30. I immediately noticed 4 differences:
1) It started quicker. It always started quick, but after the change, it was even quicker.
2) It started quieter. My truck has a 4.9L straight six in it, and they are notorious for almost sounding like a diesel when they first start, but after the change, the "diesel" noise all but went away completely.
3) My oil pressure was higher. My stock gauge didn't change, but my AutoMeter gauge showed an increase of between 5 and 10 psi at idle.
4) My truck ran cooler. The average running temp dropped by about 5 degrees, according to my AutoMeter temp gauge.
I'm not saying that everyone will have the same results I did, I'm just telling you what happened with me.
Is it worth the extra cost? That's up to you. I have used Mobil 1 ever since, and don't object to the cost, except when I'm at the store buying it. By the time I get home, I'm over it.
Although Mach 1 and others have had no ill effects from changing back and forth, I have read that switching from synthetic to standard, in an engine that is used to synthetic, can have negative effects. I can't back that up scientifically, though.
Take care,
-Chris
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