I had a fault with the crank case vac on one that was hard to track down. The fault was down to a small non return valve in the hoses that are down the near side of the crank case there was a tee joint and one of the pipes was smaller and it turned out the reducer was a small valve, it is closed at idle and open under load, the faulty one sometimes did not seat properly this upset the fuel trim at idle, its not on every engine code though and was not on the audi parts list for the engine code i had. There was a pt no on it. Don't know about oil use but the vacuum was way to high at idle. but only sometimes!