BMW Repair Questions? Ask a Mechanic for Answers ASAP
What weight oil are using in the engine at the moment?
Has there been any issues or any blue smoke from the back end/tail pipe?
These engines in these cars are pretty well known for oil usage - normally it quite acceptable to see usage around 1litre to every 1000miles - the fact that yours is burning it heavily within a litre to 400miles certainly doesnt sound good in fact that's seriously bad - and quite surprised to read you dont get any tell blue smoke at all even at startup or under heavy load/throttle..if it was burning that much you would without a doubt get at least some sort of blue smoke...if its burning that much then the emissions test would be somewhat high or fail..have they done that yet.... i note you have done/checked the breathers/ccv/oil sperator valve - have you actually 100% sure there are no leaks on the engine itself..especially around the rocker cover/oil filter housing can be areas that are common for leaks...but like i say above these engines can be problamatic for oil usage and results around the incomplete and or improper break-in of the engines when synthetic is used exclusively during the break-in period...Obviously the routes to solving this is as you mention in regards ***** ***** rebuild - replacing the rings/oil rings/valve stems/seats etc..but is is a hugely expensive task but it would likely solve the problem to some degree considering the cylinder walls could be suspect - sometimes it might be even less expensive to just put in another engine short block and just transfer all the ancillaries over from the old engine to the new...or use a slightly heavier weight oil to help reduce the consumption and live with it...these are about the 3 options open to be honest..