My car used to use about 1 litre to 300 miles, smoked on the over run, was told by a reputable tuner that 2wd piston design was not as good as later ones and most show wear at 50k miles, mine had done 63k miles, did the usual turbo and valve guides and seals, but found out that it was in fact bore wear, it had 0.008" of wear, new 4wd pistons and a rebore and now it don't use any at all, do approx 1200 miles a year in it and never have to top it up, so if its running fine don't mess with it and just wait until the consumption is really bad then do the lot, as could be a small amount from all 3 areas due to general wear.