yeah sounds like its burning oil
that would account for the decrease in oil and the smoke
depending on the colour it may not be too bad. The motor may be 'breathing' a little more running more boost on an old engine, therefore you have a bit of oil going through the intake and getting burnt which could create a whitish cloud of smoke
or, the rings may be shot which gives the smoke a bluish tinge.
Have a look at your intake piping to see if theyre coated in oil. If so, a catch can would be a good investment.
If not, get a compression test done firstly, without oil in the cylinder bore and secondly, with a little oil squirted into the cylinder. If compression increases with the oil, then you know that the rings arent the best
But first, tell us what colour smoke you see