a engine reggardless of age should not use anything but fuel/desiel.
you will probably find your valve stem seals to be worn, when your going down a hill in say 3rd does it get a bit smelly or blow excessive blue smoke?
if thats the case u may want to run thicker oil in it or replace the steam seals.
other wise u got a nasty oil leak, if it was to leak 4litres over 3500km's then I'm sure you would no about it already lol.