Castol is a semi-synthetic AND NOT FULL SYNTHETIC - and due to a loop hole is called full synthetic, following a lengthy lawsuit. Castrol's origins hark back to castor bean oil and generally speaking isn't fully synthetic. However i've had several laps of a local race track (10+) at 135 degrees celcius (unbeknown to me is complete and utter engine suicide) and the 1g-fe copped it no problem, and still makes similar power to what it did when i aquired the car 60k miles ago. HOWEVER, the 1g-fe is a really, really, REALLY conservatively built and 'tuned' engine.
At a recent track meet, my IS saw over 6 hours of continuous rev-limiter-bashing treatment by me and 5 other drivers over 350km of track work and the thing NEVER missed a beat. It still starts no problem and pulls as hard as it did from day one. Notwithstanding HOT australian weather conditions. It ran the whole day on castrol edge with no problem.
However though my DEFI oil pressure gauge, i've noticed quite a large drop in oil pressure in all driving conditions using Royal Purple Fully Synthetic oil. A drop in oil pressure relates loosely to a higher flow rate or viscosity... which is essentially what you want from an engine oil. Thickness or thiness doesn't matter, viscosity is the key.
My point is, the 1g-fe will be bullet-proof regardless of which engine oil you use, but if you want the best for whatever reason, castrol edge is not what you should use.