It is VERY common for manufacturers to buy products from their "competitors" in most businesses.
The fact of the matter is, "Made by" company "A" does not mean that it is the "Same as" another product sold by company "A".
The base stock bought by Amsoil, may or may not be IDENTICAL to the base stock used in another Mobil 1 product.
Then we go to the BLENDING of the product.
I am sure that Exxon Mobil buys some of the products used to produce Mobil 1 from other companies, and it does not make their product less that it would otherwise be.
The folks here, who know far more than I do (I learn from reading what they say), have stated repeatedly that the quality of the end product is a sum of the base stock and the additive package.
As the end user, I only care about the product in the container.
Gasoline is the same, one company's refinery supplies most all brand stations, again, it is the additive package that is added to each brand's "base stock" that makes the difference.
The tanker truck has a number of compartments, and each station is supplied with the product that matches up with it's brand.
Chevron - Techron
Shell - Vpower
BP- Invigorate
and so on.
Of course, there is less variation of the "base stock" of the gasoline between brands in the SAME market than there is in motor oil, but the point is the same.