Regarding the referenced plug it IS at the bottom of the sump left side. No external oil tank however. I'm happy it is a wet sump. I don't need or want the extra complication. But it did take a LOT of oil.
On the MV8 oil type post, your own chart shows 5W-50 as one of the Duratec oils. Photo of my manual page below and another manual I found online show the same. To say the Duratec is not designed to run that viscosity is your statement but makes no sense to me. 5W-30 is for the Sigma. As long as the car continues to perform well with this oil I will continue to use it.
Manual 2.pdf