I had the same problem last year and decided to fix it. It was intended for a top-overhaul only, but when they removed the pistons, they found that it had to be replaced. So, what was to be a minor overhaul turned major with nearly every moving part in the powerplant replaced.
Funny though, I noticed the oil soaking up mysteriously AFTER it was changed to fully synthetic oil.
After the major overhaul, its filled with semi-synthetic with periodal oil changes at 10,000 km mark, and now the oil hardly evaporates.
No oil additives added.
ps. the only major replacement in future would have to be the a/c compressor. Its starting to sound a little louder than usual.