Okay, thanks guys, thread concluded. The consensus seems to be synthetic for the engine, at least. I found and read a 2005 paper by a lubrication specialist last night at the USA Sportster site which strongly suggests Mobil1 15-50 is a very safe bet, in terms of adequacy over the long term plus ongoing easy availability. You see, Harley express a caveat in their owners manual that whatever you decide to lube the bike engine with, stick to it in order to prevent possibly nasty interactions between different oil types and their various ingredients.

Smack wrist, Mac for that long suffering gearbox! And a big elephant stamp to critter and Trax, + thanks also lads for chiming in. Promise I will try to stay safe and not kid myself any more (than necessary) that I'm still 22...

J.