There is no back door. And, it is not "pretty easy." The necessary $50-million to fund even an impaired topping unit, like the one MMEX has proposed is more than chump change.
"American light sweet crude" is the easiest feedstock to process. However, one has to understand that what comes out of the ground in West Texas varies over the whole of the Permian Basin.
The API, TAN, sulphur content, all vary depending on the sub-basin, producing structure and formations, and even the well pad the crude originates from.
MMEX provided a single assay to VFuels, and the design indicates a very narrow envelope around that assay. A real refinery (or even basic topping unit) has to be engineered and built to cope with the real world, in which there is a lot more variance in the feedstock.
High TAN will destroy the systems in a refinery, unless the feedstock was properly treated. API outside the design window has a variety of impacts - wasted raw material, imbalance in intermediate output are among the many problems the operator will face.
A shift in sulphur balance, salinity, water content, all impact operations.
What many here fail to realize is that "WTI" is a synthetic, economic benchmark - it isn't actually what comes out of the ground.
MMEX has failed to realize that. Like many other failings, MMEX (if it ever got a topping unit built) would go further broke trying to run said topping unit, because they failed to account for real world conditions.
Its pretty easy. They are going in through the back door. Its easier to ask for forgiveness than permission. American light sweet crude is where it is at. Not blood soaked Saudi or communist venizualian heavy crap. Peer revue is essential and it has passed.