I can't say if the Upper Oil Pan has a Gasket or not, but if Oil is leaking from there into the bell housing, it would NOt necessarily affect your gearbox! The reason is due to your gearbox (Transmission) and your Transmission bellhousing being separated by a metal Gasket. If any Oil were to drip into the bellhousing, which itself houses the toothed Transmission input Shaft (the teeth of which are attached to the Torque Converter which itself spins as a result of being attached to the flywheel, which itself, obviously, is attached to the Rear Crank Shaft) Torque Converter, and flywheel. Unless the oil were to pass into the Gear Box via the input shaft Gasket, it would only spatter in mist form in and around the bell housing and eventually drip from the removable rubber Grommet at 6 o'clock. As such, this would NOT cause "slipping and jerky Gear changes", as it would only cause a mess on your garage floor as well as cause your Exhaust Pipe to billow white smoke if the leak was more than acceptable. Ask me how I know!!! A misting of Oil in the bellhousing would not effect shifting, that would be an inner transmission Gear and/or Valve Body issue. A more likely reason for oil dripping from the aforementioned grommet would in-fact be a Rear Main Seal leak, as you have deduced.

As for doing the upper Pan job, it could theoretically be done without pulling the engine, but you would have to jack the car up pretty high and support the engine from above while also supporting the Steering rack and the whole front suspension from below, disco the front Subframe (6 bolts) and lower the entire front suspension. Before doing so making sure to also disco the Steering Column from the Rack! If you were to do this, it would afford you to also be able to swap out the front swaybar Bushings FYI!!

I just took these Pics today as I'm in the middle of doing a Rear Main Seal job myself! I saw that the flywheel had surface rust so I plan on treating it with some rust arrestor (Permetex) and spray it with rustolium. I'm also replacing the DriveShaft Discs, Rear Tranny Mount (the beefy Black Series!) rear Subframe Bushes, Hubs,Flanges, Rotors, Shocks, AirSprings. Happy Days ahead!!

Note:WIS Calls for you to support the engine below the pan to inhibit the engine from leaning rearward and possibly kinking the Airmatic Lines. Keep in mind that I had just previous to this replaced both engine mounts which added about an inch height! In retrospect, I definitely would have installed those mounts after having removed the Transmission as it would have meant the engine would have been that much lower, but then again, my Tranny jack only just cleared the front bottom part of the engine to allow me to clear the Tranny Driver side in front of the wheel!!!

See that pic below with the bottle jack topped with a square 2x4? As I was removing the Bell Housing Bolts, I started from the top, the last bolt was the Driver Side bottom and as I was removing it by hand, I could see the engine shift forward towards the Radiator NOT the Firewall??? So I paused and got up an jammed both a piece of 4x4 and a 2x4 between the Passenger side Valve Cover and the surface on the side of the fan to support the engine from further shifting that direction! Glad I did because after taking out the last bolt, Those pieces of wood were the only thing keeping the engine from jamming up against the radiator!! Upon install, I'll have to reposition that bottle jack to a spot forward of the engine mounts to angle the engine more rearward!

M156 Trans Bell Housing with Input Shaft and its Seal at its base.

M156 Torque Converter mounted to flywheel ex-situ. See the teeth of the Input Shaft. Also, REALLY IMPORTANt, notice the slots at the end of the Torque Converter!!! Those must line up (i.e.-click in place ) with opposing teeth which can be seen at the base of the Tranny Input Shaft. Make sure these line up when remounting the TQ to the Tranny upon reinstall otherwise Big Trouble when you try to attach to the engine much less if you try to start the car if you can even manage to attach the Tranny to the engine!!! WIS Mentions this briefly along with showing you pictures of a measuring block that you have to conform to (as if???) to be certain of the TQ being fully seated before reattachment of Tranny to engine!! What it fails to make clear is the importance of these slots lining up with the opposing teeth otherwise the TQ will not Seat properly and, as such, will stick further out!!! Sorry for not taking a pic of the teeth at the base of the Input Shaft.

M156 Tranny and TQ & Flywheel

Crankshaft Rear Face, Seal not seen as it sits behind the Plate as seen in photo.

See the Trans Oil Cooling Lines capped and pulled to the side so as to not be mangled when dropping the Tranny!!

TQ and Flywheel

Another view, that gold stuff is High Temp Shielding (1100 degrees) to hopefully extend the life of the engine mounts! I also covered the entire bottom of my Intake Manifold with the same stuff

Proof of my W211 M156 E63!!!