Could be any of the above. You take your crude, you take a fraction of it by using a distillation column to separate it out by molecular weight.
That fraction contains paraffins, it contains napthas, and napthalenes, but all with a fairly narrow range of molecular weight.
You can get narrower ranges of weight with more distillation. You can get different ratios of paraffins to napthas to napthalenes by using different source oils, or with additional processing.
There are some other compounds that come along for the ride, some of which can be good and some of which can be bad. Removing them either means changing the source oil or using additional processing.
For the most part, a lot of sour crude gets used for making base oils, because the sulfur isn't a problem for it, whereas it is a problem for making gasoline.
No, but depending on what the characteristics you want in a base oil are, it could be substantially less expensive to use one source of crude rather than another.
Man, it's been thirty years since I took a tribology class. But this is basically a measure of cohesion; how much the oil sticks to itself. There is a word for that, which I no longer remember.
I suspect that people are using this as a rough judge of viscosity since they don't have a better one. As the viscosity improvers break down, the oil will tend to get thinner.
Probably gasoline, because that is where the demand is. A lot of heavier oils wind up getting cracked, to break long paraffin chains down to shorter ones to meet market demand for gasoline.
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