Well, that's not really true....lots of people were taking bundled Mexican Mayan heavy (about 22 degrees API, slightly sour) in order to get the Reforma Light (about 43 degrees, and sweet). It's just a matter of the yield being a lot more valuable from a barrel of Reforma Light or Louisiana Sweet Light than from a sour or heavier crude.
Canadian heavy Lloydminster is so dense that people have to drill the lows to look for it -- it's about 12-16 degrees API ; and the Athabaska Tar Sands product is about six degrees. The Venezuelan stuff is in between the Athabaskan and the Lloydminster oils, gravitywise. Very heavy, but just keep adding hydrogen and you've got yourself a very refinable barrel.
Some of the Louisiana and northern Mexican liquids co-produced with natural gas (sometimes the reservoir is a single-phase fluid intermediate between gas and oil, and it phase-separates on its way out of the hole, as pressure is released, like Coke fizzing), usually called "condensate" (accent on the first syllable), is transparent pale yellow or white and has API gravities north of 60 degrees. That will burn in your gasoline engine -- though you wouldn't, because it would ping a lot and the co-produced metals, including e.g. vanadium (a hardening agent) would cause your rings to harden up and scuff your cylinder walls, and also to become more brittle, possibly cracking at higher RPM's.
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