THe Castrol Formula RS is a real synthetic unlike the new Magnatec, as its a group III. Also, one of the GTX's (in Europe at least) is also a group III, so be careful! You CAN'T go wrong IMHO though, w/ Castrol's Formula RS. The only Formula RS we can buy here in the states is the 10w-60. Whoopie! What am I gonna use that in @ $13 usd/qt?
The link that Joee provided is for Castrol R 0W40, didn't say anything about an RS.
Unless we see an MSDS sheet, I am going to assume its a majority Group III base with low levels of PAO and maybe some esters. In other words, without the MSDS, it appears to be a blend.
I would suggest that either its is a re-named product (i.e. RS-the 'S') or, its a typo. And yes, I still say its a real syn. because of the way oils are marketed there. I know that in Germany, where they are required to specify (ie. HC Synthetics or Full Synthetics) type of oil, its listed a a full syn. and not an HC syn. Of course this may only be in Germany. However, I do ask this: "Is the Formula 'R' available in a 10w-60 too.?" If so then, this would probably be the formulation as sold in Europe. Perhaps, and perhaps not.
Yup 10W60 and 5W30. I'm surprised it's 5W30 and not 10W30 as Mobil Oz said 0W30 and 5W30 are useless! Still Castrol must think differently. The oil is about A$85 for a 5.28 US quart (5 litres) jug. Thats more than M1 0W40 and I know which I prefer.
You will note that the kinematic viscosity @ 100C is 12.9 centistokes, so this is barely a 40wt grade. This is why the HT/HS is only 3.6. Cp.
This is typical of many of the 0w-40, "Euro" synthetics intended for passenger car use. They are formulated just thick enough to meet the ACEA A3/B4 requirement of 3.5 Cp for HT/HS viscosity. The 0w-40, Mobil 1 Tri-synthetic had a HT/HS of 3.55 Cp for example.