I used to work for Castrol. All oil manufacturers, including premium formulators like Castrol, on the automotive side of the business will use an additive company package from an additive company, such as Lubrizol, Chevron, BP's Adibis (sold to Lubrizol), Mobil, Lanxess and blend those to attain a product meeting specifications. Specifications can be industry specs like SAE, DIN, AP, Military or car manufacturer specific.
So pretty much all oil marketing companies will blend mineral oil with a bought in additive package at a ratio advised by the additive company. The additive companies will do all the specification approval tests before marketing their package. With mineral oils, in the UK there are only so many refineries left to go to; two last I know of. So and and all lubricant companies in the UK will be using limited options on mineral oil supply and additive packages available to meet a specific specification.
Therefore oil from company A will be pretty much the same as from company B unless it is for a highly specialised application when a unique package may be developed exclusively for a marketer, like Castrol, in cooperation with an additive company. The only caveat is if companies blend from recycled oil, that isn't as good thermally and oxidatively as virgin oil. So that is the only question worth asking if buying to a meets or exceeds specification product. Castrol would never use recycled oil when I worked there.
With blending formulations there is a range. Say oil A has an additive package from Lubrizol blended at 6.8% to meet a specification. There will be a blend range, it could be 6.6% to 7.0% to keep the specification parameters within spec on things like base number and viscosity. Your more budget blenders may well take that as an opportunity to blend at 6.6% because additive packages are expensive, so savings can be had replacing package with mineral oil.
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