Lots of good info in this thread. Adding my experience with an '04 325iT to the list:
I replaced the gasket under the oil filter housing and the valve cover gasket. Both were leaking but not enough to really affect my 1 qt/1000 mile oil consumption at 100k miles.
Then I replaced the oil separator (PCV) and tubing. It is not really necessary to remove the dipstick, but be sure to blow through it ti make sure it is not plugged with sludge. PITA job.
Then I noticed that I only saw blue smoke on startup, so I thought it might be valve stem seals. Replaced those with the head in place. Not wise, but possible.
By this time, 50k miles had gone by and I was up to a quart every 500 miles.
Plugged up both catalytic converters due to oil burning. $1000 each for California legal converters.
Compression was good, cylinder bores looked good with a boroscope.
I did NOT want to ruin any more catalysts.
Replaced engine with a junk yard unit. $1800 for engine, $1000 for clutch and other parts that were easier to replace with the engine out.
Oil consumption now is about 1 qt every 5000 mi.

IF I HAD IT TO DO OVER
I would try ATF and every other snake oil available in the oil to try to free up stuck oil scraper rings. I was about to throw away the engine, so I had nothing to loose other than risking catalyst damage.