Thanks for all the replies. I see that its partly created by just a lower market share in the us but also because they where/are known as a marketing oil company. I can kinda agree on that part, also because of the famous castrol vs mobil lawsuit.
Bmw is a famous brand and i guess a lot of oil manufacturers would be proud to have their brand on bmws oil filler cap, however as some suggested maybe this partner ship with bmw also gave them some bad marketing instead of a good one because of the problems there where. (which possible where not their fault)
The early 2000s bmw 6 cyl and v8s where notorious for sludging and creating varnish, if this had to due with design, loads of short trips, too long ocis or maybe the oil who knows. But this was indeed something that maybe became a little bit of a bmw/castrol myth.
Also all the normally aspirated bmw M engines where known for premature rodbearing failures. These cars where specced for castrol 10w60.
From what i personally read bmw designed the tolerances too tight.
I used castrol for one 15k oci in my bmw and i guess it did what it should which is not seizing the engine. However i did notice (From the engine leaks/puddles) that the castrol oil is one of the few oils that turned reddish, if i wiped it with a cloth it would look almost blood/limonade like.
The pic is a pic from the internet but i noticed that on a lot of castrol era bmw's the valve trains seem to have a reddish varnish on it, i guess this had to do with how the add pack colours under heat and also causing their reputation for not delivering engines as clean as mobil/shell engines which looked like they where machined the day before.
I will probably switch to castrol for all my gear/diff oils because they cover almost all types from cars to machines and are one of the few that offer them all in the netherlands, shell for example is much more difficult to get because most of what they offer starts from 20 liter jugs.
Castrol offers it in 1 5 or 20l jugs.