Originally Posted By: SIXSPEED
He indicated since Ford specs a 5W50 for the 5.4L, their recommended oil is 5W50. He is not sure why Ford recommends such a high viscosity and sees no issue with using an oil with a little lower viscosity.
It's likely one of those scenarios where he can't really say what he'd like to say due to liability. He can't outright go against the manufacturer's recommendations, for better or worse, because of the small chance that something might happen and he and Redline would be on the hook for it.
You may have to make an educated decision on your own for this one. Redline's oils are very shear stable, and like d00df00d said, as long as you're not diluting the oil with fuel, then you may be okay with the RL 5W40 which has a higher HTHS than MC 5W50.