I dont want to be the bearer of bad news but i had the same situation with my R. I had always used synthetic oil and i take good care of her, hell i dont even drive that hard very often. So was stumped as you seem to be, i did a compression test and they were within spec so to make a long story short i had a burnt valve on the number 3 and stuck top rings on the same cylinder. But no visible soot or any of the normal tell tale signs.

As for the validity of this full synthetic oil burning more frequently can anybody offer anything concrete?