i've used Bardahl 1 & 2 oil treatment for my 13 year old Wira workhorse and found it to be a good match if i were to use mineral & semi-syn engine oil (apart for Magnetac which i believe do not require any more additive)...
for fully syn, Bardahl has no effect i were to use Mobil 1 & Syntium 1000 but still give good results if i were to use Helix Ultra..
but now, i'm now trying Tufoil and wow, it is much better than Bardahl where my oil consumption problem has completely stopped!!...
reason i switched to Tufoil was investigate whether it could lower my FC but the results were very marginal, ie. less than 1 - 2% (probably cos i'm already on additives previously)....
but my morning start vibration (my carbie choke already 'kong') and oil consumption problem has completely gone which i did not expect for such results....
i'm currenly using fully syn Penzzoil Platinum and as you guys know, this oil will evaporate very fast for old workhorses like mine but with the Tufoil, this problem has been resolved...
morale of the story, experiment with a few products and you will know which is best for your car