Mobil 1 uses a 100% completely synthetic PAO (Poly Alpha Olephin) Group-V base stock. Mobil invented this technology back in the fifties, and they have always used the 'true' synthetic formula.
Castrol 'Syntec', as originally sold in America back in the late eighties/early nineties, was NOT a true synthetic, even though it was advertised as such.
The Castrol product used a group-III/IV base stock which is derived from natural petroleum/crude oil sources.
The lawsuit brought by Mobil against Castrol resulted in the decision that Castrol was indeed allowed to call thier product 'full synthetic' based on the fact that Castrol claimed it's base stocks recieved further refining processes, at the molecular level, than did 'normal' conventional base stocks.
This very, very refined base stock very closely approched the performance of a full PAO synthetic, so the US 'guv' decided that (paraphrasing here)" If it does the same thing, whats the difference"?, and allowed Castrol to call it synthetic.
The European government does not allow Castrol to sell this product as synthetic there...it carries a different name.
In ExxonMobil in-house tests, the Castrol product is very close to the Mobil 1 product in all areas except:
1)Phosphorous and Sulfer content, which are the main contributors to sludge formation. It comes from the crude oil, and is extremely difficult to remove completely. Mobil 1 has ZERO of these components, making it almost 100% resistant to sludge formation.
2)Mineral-oil is very susceptibale to high-temperature oxidation, creating...sludge and varnish. It also reatins heat and reduces lubricity. Mobil's PAO technology is almost 100% resistant to high-temperature oxidation. Once again, no sludge formation...with better gas mileage to boot!
3) Mobil 1 uses less additives overall (by percentage), meaning more actual oil 'in the mix', creating a cooler running engine, and producing better gas mileage.
And no, I am not on ExxonMobil's payroll, I just know whicjh is a better product overall.
If the Castrol product was a little less expensive thasn the Mobil 1, that would be OK, since it is far less expensive to produce.
Unfortunately, Castrol markets for near the same price as Mobil 1, while they have a much larger profit margin.
I'll stick with Mobil 1
Interstingly, Amsoil, who used to buy thier base stocks exclusively from Mobil, now buys thier base stocks from the lowest bidder, and are consequently now using G-III/IV base stocks in thier 'synthetic' oil.