Motor oil is always a little bit of black magic. Mostly because the ingredients for each are a tightly held secret. With all the ad hype, it's hard to pick out a good oil. Here's my opinion, though:
Synthetic oils will handle abuse better than dino oils, especially heat. The additive package is usually more stable, allowing for a longer drain interval. And that's good, because they're more expensive.
The trick is picking out a good synthetic. The first thing you need to know is, what is a synthetic oil? In the beginning, a synthetic oil started with a true synthetic PAO base stock. A manufactured molucule, derived from natural gas. Semi-Synthetic oils started with a group III or IV base stock, which is derived from standard dino oil. Synthetic blends had a mixture of base stocks which varied from one oil to the next.
Fast foreward to 2008. Due to an interesting trick by lawyers and ad men, a synthetic doesn't mean what you think it means, or what it used to mean. Any oil that has been formulated to meet certain performance charectoristics can be called a "fully synthetic" oil. Even oils that start with a group III or IV dino oil can be called fully synthetic, as long as they meet the performance standards.
AFAIK, there are only three oils on the general market that are truly synthetic (started with a PAO base stock): Amsoil, Royal Purple, and Redline. That's it, only three. Notice that Mobile 1 is NOT one of the three. Mobile 1 is actually a semi-synthetic. It's not a bad oil by all accounts; it's just not what you think it is. Now it's getting cheap enough, I might consider using it again.
Now, how much oil performance do you really need? Royal Purple and Redline are probably the very best motor oils generally available. I use RP in my cobra, since it see's a lot of hard track miles. It's expensive, but worth it for this engine.
But, none of my other cars use it. Why? They just don't need that kind of performance for a daily driver. Many people here claim M1 is an excellent oil and they've never had any oiling problems while using it. M1 is a Group IV oil. That means that any "fully synthetic" group IV oil meets the same performance charectoristics. So, why pay $5.95 form M1, when I can get Penzoil, QS, Castrol, etc for $4? My daily drivers get QS. Probably more protection/performance than they really need, but reasonably priced.
Flat tappet cams have a real problem. As has been said, the diesel oils have been reformulated, so they don't have any zinc in them either. No matter what oil you use, you MUST include a zinc additive, or your cam won't last long. Comp Cams and Crane Cams have their own brand. GM used to sell one, but they dropped it. And I believe STP is mostly zinc, but I can't prove that.
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