First work out where the oil is coming from
Owners describe the same early signs for most of these leaks: a burning oil smell after driving, oil drips under the engine, or oil residue on the subframe or underbody panel. The hard part is the location. Commenters on the rear main seal page warn that oil from the oil filter housing, oil pan or valve cover often collects at the same spot where the engine meets the transmission, so a leak that looks like a rear main seal may be one of the cheaper gaskets above it. Burning oil with no leak is a different problem, and a misfire from oil in the plug wells is in the BMW Rough Idle and Misfire: Causes and Repair Costs. Confirm the diagnosis before approving the biggest job.
This guide is built from the repair reports BimmerQuote has collected and cited. It shows what owners were quoted or paid, not how often each leak happens.
N52, N54 and N55: oil filter housing gasket
The oil filter housing gasket is the leak that owners describe as dripping oil onto the serpentine belt. The signs are a burning oil smell, oil on the belt area, and a belt that slips, squeals or shreds, and in the worst case the belt can be pulled into the front seal. Prices spread mostly by scope, gasket only versus gasket plus belt, tensioner, pulleys and coolant parts, so ask for an itemized quote. Several commenters stress priming the oil system before starting the engine after the job, especially on the N55. See Oil Filter Housing Gasket (OFHG).
N52, N54 and N55: valve cover and oil pan
The plastic valve cover can warp or crack, in which case the whole cover is replaced. Owners describe oil seeping around the cover edge, smoke on the exhaust manifold side, and oil in the spark plug wells that causes misfires. The main reason prices differ is whether the quote is gasket only or a new cover. See Valve Cover / Valve Cover Gasket.
The oil pan gasket shows up as oil drips under the engine or residue on the subframe, and on most cars the front subframe has to drop. On E9x rear-wheel-drive cars the pan bolts are aluminum and torque-to-angle, so a torque angle gauge is needed, and xDrive cars cost more because the front axle is in the way. Engine mounts and the oil level sensor seal are common add-ons while the subframe is down. See Oil Pan Gasket and Engine Mounts.
Rear main seal
Oil dripping where the engine meets the transmission, oil on the bellhousing or under-tray, and a burning smell if oil reaches the exhaust point to the rear main seal. The repair page covers the N52, N54, N55 and one M4 with the S55. There are two seal sizes, and a BMW shop foreman advises replacing with the size that came out. On manual cars commenters say to do the clutch at the same time because the labor overlaps (E9x / F3x Clutch Replacement). See Rear Main Seal.
N20 and B58
On the N20, the valve cover job is longer than on the six-cylinder: FCP Euro's guide has you remove the intake side parts, the fuel injector harness and vacuum lines to get the cover off. A BimmerFest member notes the N26 valve cover gasket is covered for 15 years or 150,000 miles in PZEV states, so check the warranty before paying. The N20 shares the Valve Cover / Valve Cover Gasket page with the sixes.
On the B58, owners describe a burning oil smell, seepage at the cover edge and oil in the spark plug wells. The cover is normally replaced as an assembly with the injectors coming out, and the billed hours in the reports range from roughly 6 to 13.5, so ask for the hours and labor rate in writing. Some B58 cars may be covered by an extended warranty or campaign, so check by VIN. See B58 Valve Cover Replacement. The plastic oil filter housing on the B58 and B48 is more often a coolant leak, covered on B58 / B48 Oil Filter Housing (Coolant Leak).
Rear differential axle seals and oil burning
On the X3, oil residue around the rear differential axle shafts is usually found as a seep on a lift during tire or brake service, not as drips in the garage. A forum member says a set of one-time-use bolts must be replaced in the rear end for this job. See G01 X3 Rear Differential Axle Seals.
Burning oil without a visible leak is a different problem. On the N63 V8, owners describe blue smoke and about a quart of oil every few hundred to 1,000 miles, which the repair page ties to the valve stem seals (N63 Valve Stem Seals).