I read an article once about a man in a state near me that had a Chevy pickup that he used to deliver news papers. He put over 1 million miles on that vehicle, changing the oil every 2 weeks because that's about how long it would take him to put over 3,000 miles on it. He used standard dino Pennzoil, nothing fancy. The place where he got his oil changes informed Pennzoil and the local paper about this truck. Pennzoil bought him a new Chevy too, used his old one for research. Only major component he had to replace was a transmission at 300K miles. Second one lasted over 700K.
Like oracle stated, it's all about how well you treat it. Not necessarily what you put into it. Synth oils do have some nice features, but their are not a needed thing by any stretch of the imagination in a normal vehicle.
Drive it nicely, and keep up with the recommended maintenance and it'll last as long as you want it to.
Also, considering the platform the Theta's are built on, I'd treat it really nice if you want decades of service and many 100's of K miles. There is a major difference in the entry level vehicles like the Thetas and much more expensive vehicles IMHO. Not that I'm saying the Theta's are bad, they are not. But it is what it is. The Silverado's and Econoline's of earlier posts are built quite differently. They are expected to be treated rough and used to an extreme. So they are obviously quite a bit tougher.