hi, what turbo you using, what boost are you roughly running? and what breather system have u got?
despite what many people will say, cossies do smoke on over-run its normal.

1. However some simple checks for the turbo are to take off the boost pipes and checking for oil,and also check the throttle body by opening it and then puting your finger in there or looking to see if any oil is getting in.

2.The top end was rebuilt but at what mileage and were the valve guides changed or just the seals?

3.the oil seems a little too thin as back in the 80's cossies used 20/50. so as suggested try a different oil such as mobil 1 15/50 or millers or silkolene etc.

500 miles to half a litre is a little high but not uncommon for a cossie.