The patient is a 2012 F25 35ix N55 engine. 105k miles.
My level of experience: experienced diy-er. The most relevant experience was replacing the clutch on e46 330 manual (twice) and oil pan gasket on the same car. So this may give you an idea of what time you may spend on it.
I spent 2 weeks: 2 full weekends and 2-2.5 hours in the evenings on the working days.
Another half a day I spent on building from 2x4s and 4x4 an engine hoist and subframe lowering frame. All of them are visual copies of respective BMW tools from ISTA Rheingold. Both did fit like a glove.
I got everything from fcpeuro (including all nuts and bolts, which use angle torque or self-locking nuts, per ISTA). Everything came to $228 (then another $162 as I broke tensioner arm ball joint in the process): oil pan gasket, 2 driveshaft seals, 2 driveshaft o-rings, Loctite SI 5970 gasket sealant, oil pan bolts, 2 drive shaft nuts, 4 propeller shaft bolts, 1 steering shaft bolt.
How it all started (and why I got Loctite sealant): see
https://x3.xbimmers.com/forums/showthread.php?t=1865579.
My source of information was a two-part video from youtube by 50sKid "E83 X3 Oil Pan Gasket Replacement DIY - Part 1" and part 2 (which I did not watch) and ISTA Rheingold.
Now is the most valuable info: that youtube video is almost 100% accurate, almost nothing changed between E83. I will highlight below what did change. Also, do not follow ISTA recommendations in lowering the subframe, it’s not needed, it's too much bother.
Outlining major steps:
1. Remove intake silencer / muffler and intake duct, ignition coil covering.
2. Screw in the towing hook (I borrowed one from 330 as the stock one did not fit);
3. Install self-made hoist and support the engine.
4. Remove bottom protection, including one over the transmission pan. I also removed the subframe reinforcement plate.
5. Drain engine and front differential oil.
6. Look at the video on what to remove in suspension: the only two things you need to remove/disconnect is the anti-roll link from the bar and wishbone to the subframe bolt and nut. But before that remove the drive axle hex bolt.
For whatever reason I decided to disconnect the tension strut from a ball joint with a disconnect tool and bent it in the process (I should have kept the nut, applied enough tension and tapped with a hammer instead). Nothing tool-ish or bruteforce-ish worked, so I had to remove the tension strut from the subframe (roughly $20 new bolt and nut) and a new ball joint is ~$50. Only then I could hammer it out.
On the other wheel the tension strut stayed happily on.
7. Now, the wheel hub easily pivots and the outer end of the drive axle is free (it's California). This is different from E83, which has splines and inserts into the wheel hub.
8. Knock out the axle as the video shows. I used a piece of oak door trim (did not dare to use a steel pry bar). Easy job. Proceed to the other side.
9. Disconnect propeller shaft from the front differential. No need to remove as ISTA suggests - I hang it from the top on the rope coming through the engine bay. It’s a good idea to mark with a sharpie the shaft flange to the differential flange, so you put it exactly as it was - sometimes the propeller shaft is exactly balanced.
10. Now, undo two bottom front differential bolts. Keep them in on a couple of threads. Undo the transmission cooler lines bracket on the oil pan on the transmission end.
11. Remove the bearing supporting bracket from the passenger side.
12. Disconnect the lower bolt from the steering shaft. Do not rotate the steering wheel after that. The joint will pop out later when you will be lowering the subframe. Disconnecting will give you more space later. In the video, it was not disconnected.
13. Disconnect 3 electric plugs from the steering box (spent like 20 mins afraid to break the plastic), unclip all wires from the subframe (clips break, be ready to purchase new ones on amazon). The nasty one is the middle plug wire, which goes up under the driver's side wheel arch cover: it clips in two places to the subframe, I did not see the second one and almost snapped it when first lowered the subframe.
14. Disconnect the plastic frame, which supports a coolant pipe, on the front of the subframe - 3 torx bolts, one of the driver's side is hard to locate. Make sure that you never lower the subframe too much to stress that hose (as it's passing under the anti-roll bar). No worries, you will have enough space.
15. Undo two engine mount bolts, raise the engine to 10mm.
16. Disconnect the lower end of the intercooler pipe (driver side). Make sure you jam it so it does not clip back in. This step is not in the video! You need it to access the forward facing transmission cooler lines bracket bolt on the oil pan.
17. Partially undo the underbody protection (or completely if you can) on the right and left, so you can pull it down as the rear ends of the subframe are under it. Better even to do it at the very beginning.
18. Support the subframe with your perfect 2x4 frame and trolley jack. Undo the 6 bolts and remove them.
19. I hoisted the subframe using the Home Depot $9 for 4-piece double-hook tie-down straps. Hooked one end to the coil spring, another to the front and the back ends of the subframe; both sides. Because you later need to remove the trolley jack.
Now, hoops jumping starts.
20. Lower the subframe for ~4 inches. Lower the engine for a couple of inches. See if you have enough room to pull up that intercooler pipe and access the cooler lines bracket bolt with the tiniest socket wrench you have. I spent tons of time on that, it was really inconvenient access.
21. Make sure you can see the upper differential bolts. If you cannot, lower the engine more. Use multiple extensions to reach those bolts. Undo and remove.
22. Raise the engine again. You need a room between the subframe and engine mount to maneuver the differential out. I could not do it as in the video, with the subframe in place.
22.1 Pry out Drive Axle Shaft Seals from the differential and Bearing Support bracket with a screwdriver. Clean surfaces; wet a bit with oil and install new seals. To spare you time, here is what I sourced at The HomeDepot. The gray one presses in the seals. OD is 69.5mm; ID is 60mm. I found it the best fitting, from the electrical department, while same 2'' pieces from plumbing department were a tad bigger. May be not a big deal as you need to install them flush, but worked perfectly for me. Tap the other end with a hammer.
23. Lower the subframe to the comfortable height, where you can still climb under, and that coolant pipe on the front is not yet caught/stressed by the ant-iroll bar. Make sure your tie-downs are tensioned and hold the subframe. Steering shaft should have already popped out at the u-joint.
24. Remove the trolley jack.
25. Undo all the bolts on the oil pan. The new oil pan gasket has 30 holes, but the pan has only 28 bolts. It likely is stuck, even after removing all the bolts. Pry it off at the transmission end (there is a niche for a screwdriver).
26. Clean it nicely with a break cleaner, or cleaner of your choice. Clean mating surface: I used acetone and a rug so not to produce micro scratches.
27. Remove that side panel, which leaks, on top of which the bearing support bracket sits. Clean it nicely. Use acetone to clean mating surfaces. Apply Loctite SI 5970, put it back, tighten all M6 bolts evenly across, then tighten to 56 inch/pound (can’t find precise spec in ISTA, but found something similar on internet - steel M6 into Aluminum). Do not fill in oil for at least 24 hours; avoid long trips for 7 days - SI 5970 instructions say.
28. At least, clean the mating surface on the engine itself; I cleaned everything I could reach on the outside as well.
29. Practice a bit installing the pan back on without the gasket.
30. Put on a gasket on the pan, get back under the car, position the pan - and I used a bungee cord loop, attached to engine mounts, going under the pan, as it’s going to take a while to align the gasket nicely to the holes, and when aligned - tighten bolts somehow.
31. Insert and tighten all bolts manually, feel by hand. No cordless tools. You don’t want to mess up the threads and helicoil them afterwards. Tighten evenly, then with the manual socket wrench. Then with a torque wrench by spec to 8nm. Then angle tighten to 90 degrees short / 180 degrees long. Altogether it took me 2 hours.
32. Assemble everything back. Same hoop jumping with the differential. When bolting up the subframe, position it nicely, make sure it’s fully aligned, and put in place by the trolley jack. Words of wisdom: never try to loosely position the subframe and then hope to put it in place by tightening the bolts, unless you enjoy heli-coiling. For the same reason, feel by hand, tighten by hand, finish up with the ratchet and then a torque wrench.
33. Axle nuts were a pain in the back. By spec, it’s 210 nm while my wrench was up to 200 nm (and I can’t trust the top/bottom of the wrench range anyway). And then angle-tighten to 90 deg. I could barely make it 80 degrees.
34. Wishbone to the subframe: tighten reasonably well, but tighten to the spec when the car’s weight is on the wheels (on the ground).
That seems it. Doable, the car started right away, no engine codes. You will need to do the alignment due to the dropped subframe.