Truthfully, any oil in my opinion. I've been running conventional oil for 80% of my cars life, and based off pictures of my valvetrain, and a few used oil analyses, it does a darn good job. I currently have 126k and have no plans to change.
I buy whatever name brand oil I can get at the cheapest price. For the past few years, that's been valvoline nextgen conventional since I've been able to get it virtually free after mail in rebate. Before then I ran havoline conventional oil since I was able to snag it at a great price. I've also run 3-4 jugs of g-oil bio based full synthetic oil (the base oil is basically made from animal fat) because I've also been able to get that completely free after rebate.
You can see pictures of my valvetrain here
Heres a few of my used oil analyses showing very low wear levels with conventional, and I've attached a picture of my most recent used oil analysis. The most recent analysis was run so short because I wanted to change out the oil after I finished the valve cover gasket job.
Now, even though I'm saying conventional oil, there is one more thing to note. The only reason I use conventional is because my car consumes 1-1.5 quarts of oil over a 4,000-5,000 mile oil change interval. If my car didn't consume any oil, I would run synthetic even though I don't need to.
Does your car consume any oil?