its worth remembering that an aircooled motor will always have its clearances on the 'loose' side compared to a watercooled engine. if you can control engine temps with watercooling, then bore to piston clearances can be tighter and thus the possibility of oil being burned when the engine gets hot can be reduced. 1 litre per 1000 miles sounds a little on the high side, but is the bike smoking or have you visible signs of oil consumption, fowling plugs for example or residue on the plug when you check it.
its also perhaps worth a mention that very modern fully synthetic oils can be more of a hinderance than a help in aircooled bike engines. i once got some fancy fully synthetic oil for my katana, (it was free from a guy i knew who had sponsorship from the oil company concerned) so in it went, the engine having been rebuilt just before. it was awful, not only did i have a flickering oil light, but there were several small leaks from the motor and a slight burning oil smell when it was running. after a few hundred miles i dumped it out and refilled with duckhams mineral oil and it was transformed. modern oils are super thin and just dont seem to suit older engines that are aircooled.