If I remember correctly, Castrol lobbied to change the definition of the word 'synthetic' as applied to motor oil in the 2000s.
Originally 'synthetic' meant 'non-naturally occurring, synthesized in a lab from scratch'.
Castrol got that changed to also include 'naturally-occurring motor oil that has been subjected to sufficient laboratory procedures to improve performance' or some other horseschidt.
Brand loyalty in 2023 is like leaving the back door unlocked - makes it easier to get cheated (on) by unscrupulous companies that will lower their quality behind your back to raise their bottom line.
540ratblog.wordpress.com
skip down to
"212. 5W30 Castrol Edge Extended Performance (gold bottle), API SP, ILSAC GF-6 (this was NOT listed as 6A like other motor oils with the latest ILSAC certification), GM dexos 1 Gen 2, synthetic = 75,212 psi"
After reading that blog, I'll only use Castrol for top-off, never again for oil changes.
Generally prefer Quaker State / Valvoline / Pennzoil / Mobil
1 synthetics, 0W30 or 5W30.
Once I understand that blog better, I'll consider use of 0W20 or 5W20 in the winter.