I hate to break it to you but Castrol Syntec, although being advertised as a full synthetic is actually a semi synthetic. It is essentially a base stock oil that is massaged into a synthetic like many other synthetics now. MOST major oil companies do this. Mobil1 actually sued Castrol over this but eventually lost the battle as the courts decided the end product could be classified as synthetic even though it wasn’t 100% synthetic.
That’s why sticking to brands like Royal Purple, Motul & Redline guarantee you that you are getting a true full synthetic.
NISMO_GTR - what viscosity of ENEOS oil are you using? I have some 0w-50 oil that I had sent to me but Im VERY concerned about running it in a street car but have been told it will be safe.