Looks like you are due for a bit of repair work/maintenance... I have seen countless sludged BMW engines due to the horrendous way 95 percent of owners maintain them, but if you are losing that much oil (I don't care what the manual says, but a properly maintained and perfectly buttoned up M5x/S5x will NOT lose oil!), I think that there is a few things in order...
- Switch to REDLINE OIL and start with short OCI, change filter or at least inspect regularly, and gradually ramp up the length of time between changes. I have done exactly this with every sludged motor I've owned and many more for friends, and it has never once not worked perfectly.
AFTER you do that...
- Replace the ENTIRE PCV system, but you really need to order all the "cold weather" versions of the parts, as they are significantly more durable, and I have even found that they result in quite a bit less oil making it to the intake manifold. Oh, you must replace the dipstick tube as well, cold weather version.
- The VANOS is shot, I absolutely guarantee it. The reason why I say that the sludge needs to be taken care of first is because of the VANOS. The entire unitmust be rremoved, and while it is DIY-able, it's not quick or easy.
Do not, ever, under any circumstances replace the VANOS internals w OEM ones! The best, in fact the only option that is not just another short term (20k mile) solution is the seals and other bits from Beisan Systems. I have also used the DR.VANOS full replacement units, but the Beisan route is superior. The seals, unlike stock, are not degraded by the engine heat, the tolerances are vastly improved, and I guarantee you that upon replacement you will be amazed at how much power, and economy, you gradually have lost.
Stay away from flushes, etc (Seafoam has been beneficial in minute amounts, and Redline SI-1/Techron should be used regularly), because you CAN cause catastrophic damage to these engines quite easily with that garbage... The oil pressure tanks, lifters fail, VANOS parts break, oil pump surges before dying from a clog and the pressure difference causes burst seals, and I can go on, I've seen it all).
