@Charliessmith, as a DIYer buying engine oils for my cars for 45+years, I have used Castrol in the past, mainly Magnatec when I ran a VX Cav GSI 2000 16V 4X4 (all of that was on the boot lid!), and was happy with the idea that Castrol Magnatec was said to stay coating the bearing surfaces after the engine had been stopped - though maybe all other oils did that as well but did not make a big thing about it. I tended to buy that oil from my local VX dealership at a reasonable price as if they as a VX dealer sold it I considered that it was right for that job.
Later I became aware that in all the aftermarket engine oil sales, only Castrol had started to stamp down on places setting their own prices, ie chipping maybe a £1 off a 5 litre can - but would always allow/force a "toy" to be given away FOC with a purchase - now as an adult, I did not consider that was in keeping with fait trading, so I immediately dropped Castrol from my engine oil options, this came at a time where many people were becoming aware that Castrol was trading in its past history of quality and involvement in motor sport - and opening their eyes to the fact that there were other good as or better engine oils out there as far as using in their specific car's engine.
Don't get me wrong, Castrol group do get involved with a huge amount of oil "designing" and testing but will get paid well by manufacturers for providing that service - but so will some other major lubricating oil companies.
When I moved from Ford+VX to VW Group cars, initially I started buying Quantum oils, Gold first then when it was discontinued Platinum, while fully understanding that Quantum is only a trading brand name for VW Group aftermarket service/maintenance products, ie intended never to be used in VW Group workshops but by individuals or general independent workshops supporting VW Group products - and maybe at one point Castrol was the supplier of some or all of these oils, but don't thing that the "extras" would be the same as an equivalent Castrol branded oil.
Later on I became aware that VW Group use Fuchs oils in the factories, ie Fuchs had the contract to supply factories, so I moved on to using Fuchs oils, again a quality version like Super Syn, then getting hold of the exact Fuchs oil I wanted became a bit less easy and maybe for a short time Castrol won the contract to provide VW Group factories with engine oils, but later at renewal time it went back to Fuchs, by this time Castrol seemed to have been able to place their branding on the oil filler caps on all VW Group engines - which is a blatant but maybe naïve way of getting their brand name into owners heads, a bit like imprinting that into owners heads, which is not too clever for everybody involved - brain washing in fact.
Then many people from many parts of the world were being impressed by Mobil One ESP, I had just bought a slightly used 2011 Audi S4 and its supplying dealer had just serviced it and used Mobil One ESP 5W-30, so that made me consider buying Mobil One ESP 5W-30 for all my cars as it was suitable for all of them and Opie Oils sold it at a very acceptable price.
Finally, sometimes in life we acquire "hero brands" and just don't bother to think about if they are the best that we can buy for the money being asked, then adulthood kicks in and if you are at all interested in certain things, you might do a re-take/re-think and question if some of your buying decisions are still correct, ie "am I being taken for a ride?" - that is mainly why I left Castrol certainly for engine oils, it could have been different if Castrol had quietly continued to produce good oils at reasonable prices, but for me they had become arrogant and were judging that people would take them as being best at face value - and most customers can have, if they open their eyes, have a choice, and my choice was to walk away from Castrol engine oils - that is all.
How do you with certainty fully test an engine oil against a range of others at the same level in the market in a variety of different engines under a variety of road condition - that is a tricky one as the company that pays for the testing or carries out the testing is never going to submit a report that does not place them head and shoulders above their competition - are they?
Each to their own really aided maybe by what others in a similar situation and general car DIY knowledge have worked out.
Edit:- sorry I didn't answer your last question:- do you really know if the Castrol Edge used by participants in motor sport is exactly the same as Castrol Edge that you might use in your car, and if it is, is it still completely relevant to how you use your car and its oil change period when compared with how it is used in motor sport with a motor sport specific designed engine?