in order to get THE most benefit from synthetics, you must use a PAO based one. the only ones i'm certain of are amsoil and some mobile1 oils(not all of them). many synthetics are fake and mostly dino juice.
the cost to me is well worth it. i only get under my car twice per year. once to change the oil, and a second time to change the filter half way. amsoil 0w30 claims to last for 35k or 1 year, but i'd go no more than half that(miles) or stick w/ yearly changes.
My father, brothers and I began using Amsoil synthetics in the 1970s in everything we own, and we do extend the drain intervals to 1 year. We've used the stuff in everything from 4 Cyl Toyotas to big block GM V8s and in what must be a million miles since then have had zero failures.
We start the synthetic right after break in oil is drained and continue until the vehicle is sold and have several examples of over 200,000 miles with no significant oil consumption (that is no oil added in the 1 year service life). We have tortured the oil with cooling system failures, severe towing, motor-home mountain driving and more.
We also notice the synthetic lubricants in transmissions allow for much easier shifting in cold weather (which was one of the original selling points for us.) While we agree that improved mileage seems minimal, there MUST be some given how much easier things shift and roll.
As a side note their 2 cycle oil runs at twice the ratio of regular oil (100 to 1) cutting smoke from small engines to virtually nil and giving more power too. They couldn't do that if the lubrication factors weren't much better than petroleum based oils.
Wonderful stuff and no, I'm not a dealer, just a happy customer!!
I for one, prefer the good old oil. I commute 120 miles a day with the majority being on the highway. So every 3 to 4 weeks I give the car an oil change and filter. 157,000 miles now and it doesn't burn a drop and the engine looks clean. Of course this is a 1990 car. The added cost of the more frequent oil changes is well worth the security of keeping the engine clean and running right.
I've always been a fan of synthetic oil. I can be fairly lazy and I like having longer between oil changes. However, when I bought a 2009 Subura WRX last week, they said that I shouldn't run synthetic in it for the first 10,000 miles and even then I should ask the dealer for their recommendation before I put it in the car.
The owners manual said that synthetic is just fine for the car, but that I should still change it every 3,750 miles (same recommended timing for regular oil on this car.)
I'm still within my break-in period for the car, and it's never a good idea to switch to synthetic until the engine is fully broken in. However, once I get to my first oil change at 3,000 miles, I'm not sure what's going into the car.