I've rebuilt a number of engines over the years. And the one thing I think is key in choosing a oil is heat. If you put high loads on a motor the oil is going to get hot. And it's ability to not coke is important.
The lifters usually seem to be the first items that get coked up when you put a lot of heat into a motor in my experiences.
Also, where and how the oil returns to the pan can coke things up. For instance the 2.9L in the 87-ish rangers spray oil right on the crank. Sure enough.. you'll see it very black in that area if the motor got hot.
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