Disclaimer: I'm not a petro engineer nor an expert on engine oils.
However.
If you ask people who DO know, they will tell you the same things with only slight variation so after awhile you start to get the idea.
1) Do not use synthetic oil for break in! The increased lubricity of the oil will hamper ring break in. If you want your motor to last for 1,000,000 miles and take 20,000 miles to break in, well then in that case go ahead I guess. BTW, 15w50 is only available in synthetic so that means don't use 15w50 for break in.
2) Ford specified 5w30 oil for our cars for the purpose of increasing gas mileage, reducing emissions and possibly to improve cooling. That specification has nothing to do with the mechanics of what is going on inside the motor. The SC engine does not require a low viscosity oil even when brand new. The SC engine will TOLERATE 5w30 if you go easy on it. If you run it hard I can pretty much guarantee what will happen to it over time and non of it's good.
3) Many NEW engines require 5w20 oil. These are OHC motors that need a low viscosity oil to lubricate the top end of the motor on start up. OHV motors, as a general rule do not require this. Our motor is as old school as it gets when it comes to lubrication requirements. I would not run lighter than 10w30 oil in an SC. There is no requirement in the SC engine for 5w30 oil.
4) Oil viscosity. Perhaps an engineer will chime in, but as I understand it the lighter number (the 5 in 5w30) represents the oil's true weight. The bigger number reflects the oil's ability to retain viscosity at operating temperature. This higher number is typically achieved with the use of viscosity enhancers which are NOT oil. The more viscocity enhancers an oil has the less lubricating it can do. A wide spread in the 2 numbers is not typically desirable. Keep in mind that synthetic oils by nature will comparatively speaking be capable of a wider range of viscosity with less viscosity enhancers than standard oil due to their superior molecular composition. There is a 15w30 oil available for racing. This represents a heavier base oil with less modifiers which will perform like a lightweight 30 oil under heat and load. Many racers recommend and use this weight of oil but remember, what is great for a 1300hp drag motor isn't necessarily good for a street SC that sees a wide variety of conditions.
5) Detonation. I do not believe that Ford engineers expected SC's to suffer prolonged detonation over their life times. However, that it occurs is a fact and so dealing with it in the end is our job. If you ask an engine builder or engine designer how to deal with detonation you will get a dumb blank stare as they consider you an idiot. You are not supposed to "deal" with detonation, you are supposed to eliminate it. In a perfect world we would do so. However, the reality is that most SC's detonate regularly and so we end up having to deal with it. It is the held opinion of at least one engineer and engine builder that running a higher viscosity oil than is exactly required for lubrication may provide some "cushion" on the rod bearings during the detonation events that destroy the bearings. The fact that I was able to run my motor with detonation AND worn out bearings without damage or failure lends credence to that belief. I realize that personal experience is not acceptable as scientific evidence, but some of us are having success where others are not. Take that for what it's worth.
6) I still recommend Mobil 1 15w50 for an SC motor with 80K miles or more on the bearings or one with less miles that may have wider than stock clearances.
If you disagree with anything I've said that's no problem. Do what works for you.