Hello everyone,
Thanks for all your replies

. As ever, a mix of opinions. I have much info to report now...
1) I asked my nearest Honda dealer, who recommended 10/40 semi synth, and insisted that i should buy their stock (Castrol, I think), because any other brand was rubbish. Needless to say that i politely declined their kind offer as they were far more expensive than anyone else.

2) Other local bike indies all seemed to stock Rock oil, so i did some research....
3) Rang rock oil for some advice as they have many different specs, including two different 10/40 semis

. They couldn't really tell me the difference between them, but recommended that i buy the more expensive.....
So decided to buy 4l of Rockoil 10/40 semi JASO MA for £19.50 and felt quite pleased.

Then, remembered that my Dad had an XBR for 10 years in the '90s and asked him what he used. He said that he always used 10/40 semi car oil and had no problems, and he pointed out that the XBR handbook did not recommend any special oil for wet clutches, merely that the oil should meet the SE / SF spec.
Heck, now i was feeling conned by all the dealers who had told me to buy special bike oil (Honda especially), because all the car oils i have ever used exceed the SE / SF spec and are a lot cheaper.
So, in the end, I decided to use exactly half the Rock oil, and I topped up with 10/40 semi car oil.

to be continued.........