Thanks David.
No humidity problems where I am and it'll get regular runs so synthetic will be fine.
In the late 70s/early 80s I had some friends that ran their YZ80(F I think?) on Castrol R - lots of smoke but I loved the smell of that stuff.
Anyway, at the moment I'm still thinking Castrol TTS at 50:1 (sorry mollygreen) but am still open to further advice, debate or comment.
Cheers.