I've worked in a fast lube shop for over 2 years now, and witnessing what happens to vehicles that run over 3k miles with conventional oil, I'll never use that stuff again.

BUT, it does depend on the individual driver. How you drive, climate, abuse, proper maintenance of other compnents, like coolant (which affects engine heat, which in turn affects oxidation properties of oil). So there are tons of variables to mess with when it comes to how long you should wait to change your oil. I use Royal Purple right now in my Ram Daytona. I spend 80 bucks on the change with the filter, and I do it myself every 3k. The way I look at it, I'm spending a few extra bucks per oil change to to increase the life of the engine, not the drain intervals.

I'm not saying where I work or the particular product we use because I want to stay as unbiased as possible. I'm not pitching any particular brand either. I've had good results with Pennzoil Platinum and Royal Purple(which I've used the last 3 years in everything I own now).
Those are the only two synthetics I've used. And as far as I understand it, most of the big name brands are not entierly full synthetic. They still use a crude oil basestock, its just cleaned up and modified. And they do outperform conventionals.

I have no experience with Amsoil, but Royal Purple is Fully Synthetic, no crude basestock. They claim it will pour out of the bottle at -70 degree's F. The big name brand we use only makes it to -50.

Benefits include: reduced friction, reduced heat, reduced wear, easier startups all year round(flows very well), also leaves a film on metal after shutdown, which helps inhibit rust from ambient moisture and assisting in easier starts. Also keeps parts superclean compared to conventionals, maintains viscosity which is very important. At our shop, our product guarantees it will remain a 5w-30,5w-40 or whatever you choose, for the 3k mile interval. A lot of oils breakdown due to pressure, heat and shearing of molecules, which means by the time you drain it, it coul be as thing as 5w-10 or straight 5 weight. That's no good.

Basically, continued use of full synthetic WILL help your engine last longer, as long as you don't neglect all other recommended maintenance. Hope this helps you make a more educated choice what to use or how long to spread out the drain intervals.

Oh, and make sure you just use the recommended viscosity. I.E. 5w20 for hemi's. Especially if it has MDS.(problems with wrong oil)

 

I use Mobil 1 Fully Synthetic 5W 20 and a Fram Tough Guard every 3,000 miles. It seems to do a pretty good job. I know there's probably something better out there, but that's about the best you can find around here. I just hit Wal-Mart up. lol.

 

I am very religous about my oil changes now that I do them myself....I run standard name brand full synthetic.....I drain every 3k and change filter and after running it through a good filter i reuse it for 3k more. The filter is the more important part of the equation.....buy good filters......I am happy with reusing and topping off at 3k and replacement at 6k.