I'll second Mr G's thoughts on this as my 1997 carb'd 900 does smell like it's running rich but always needs choke to start from cold, is on the button when warm whether it's stood a couple of minutes or half hour. The plugs are a nice colour and the fuel consumption is usually around 48mpg's average and more on a steady run.
I've owned it almost twenty years and it's done 35,000 miles and never uses any oil whether it's a short fifteen miles for an escape to get a cup of tea or an all day 400+ miles while on a long weekend away.
A litre every 500 miles is a lot to me when you consider the total capacity the motor holds (I've had two strokes that have used less
) so I would have thought it must be going somewhere.
I know I've read somewhere, a permissible oil consumption figure which I remember being (in my mind) surprisingly high so you might be within it but either way, it won't thank you for only doing 500 miles a year!
In my experience Ducati's do not like being left un-used for extended periods of time- seals dry out and things start to get gummed up or corroded and the best ones I have known are used regularly and have got good mileages on them.