i've just ordered some for a customer's car from classic castrol. they list rs in 10/60 on their website in 1 litre and 4 litre cans. i ordered 2 by 4 litre cans over the phone .
Just had my Griff' serviced and the garage told me its what they prefer to use in TVR. Slighter holds oil pressure better and quitens tappets a tad. Having driven it I agree, for me its better option than Mobil 1.
Cost me £47 for the oil which I think is good value.