You only recently switched to Castrol 0w-40, so what oil was in it since new? Whatever it is, it kept it clean. Thanks for the pictures, just amazing to look at.
On an older vehicle especially, going to a slightly thicker oil, and one like that which meets higher Euro specs, is smart. You could choose Castrol Ext Perf gold jug if you want, but you're fine with 0w-40 with the tougher LL-01, etc. specs.