Originally Posted By: 901Memphis
Maybe the answer is that the engine is versatile enough to use anything in between 5w20 and 5w50?
That's the case with ANY engine. Cold oil is basically always too thick. And ALL oils thin as they increase in temperature. But if you limit the maximum temperature the oil can get to, you can run whatever grade of oil you know will protect it within that temperature range. Since 5w-20 is going to be better for fuel economy (and in turn, CAFE), fitting the standard GT, which isn't designed to be tracked, with a thermal castration mechanism to keep oil temperature in check by pulling power allowed for Ford to control viscosity by controlling oil temperature. Essentially, they ENSURE that the oil CANNOT get too thin by limiting the output of the engine.
The BOSS 302 on other hand was designed to be tracked, and subsequently was spec'd with heavier oil and a factory oil cooler. So not only was Ford attempting to control oil temperature the good old fashion way (without the reduction of power output) but they also saw it necessary to spec a much heavier lubricant. This would indicate that they feel that the oil WILL get above the safe operating temperature of their 5w-20 product, or even their 5w-30 product, both of which they already had on the shelf.
This carried over to the Track Pack version of the Mustang GT, which was fitted in a similar manner to the BOSS 302.
However the answer, regardless of how happy the engine is to operate on any grade depending on oil temperature, is to run the approved product as per the owners manual while under warranty. It really IS that simple.
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Everything is dependent on what he's going to do with it and it doesn't sound like it's going to be tracked. Let's try something different here maybe meet in the middle and run some PU 5w30.
I still think it's yet another perfect fit for M1 0w40.
The warranty is dependant upon adhering to the terms Ford defined for it. There's a recent thread about somebody who is seeking warranty service on the screen for his infotainment system in his new Mustang and is having an awful time trying to get Ford to fix this $100.00 part. Do you think Ford is going to be like "oh yeah, you ran the wrong oil in your $20,000 engine, don't worry we'll replace that under warranty NO PROBLEM! And here's a rental GT1000 for your trouble!".