Offshoreonly.com - Another Oil Question

I've read all the threads about engine oil choice on OSO, and in my mind it seems Mobil 1 15w50 is the way to go. But I can't get any of it locally in the UK. I can however get Castrol RS 10w60 which is Castrols fully synthetic 'flagship' oil in Europe. (Not sure if it's available in the US, same problem I'm having with the Mobil 15w50 in the UK) . I've spent quite a bit of time trying to research this oil on the web and from all the information I have found it does seem to be a good oil. However I can't find any evidence of it's use in a marine big block.
In my mind, it should be a good choice, but are there going to be any obvious cons to it's use. At low temperature it should be slick enough to give good lubrication from startup, and under high temperature use it should hold up well, the longer chain polymers provide an even greater film strength (So I'm told). Is there anything I've missed as to why this would be a bad choice of oil?
Thanks for any advice
Matt