This has to be the most common oil leak area except for the valve cover on these engines. The gasket here is not rubber, but made of some sort of compressed fiber and they tend to start leaking after a few years. The leak from the oil pan will make oil spill from the front of the engine and oil will fly pass the whole length of the engine and on to the transmission, making oil seem leaking from everywhere. The MOT inspection might not like it either, although they usually will have more forgiveness towards classic cars. However you should not enjoy leakage from the front of the engine.
The leaking oil pan is usually not enough to drip on the ground while parking only for a day, but is surely noticeable after parking on the same spot over a longer time (like your garage). The M110 engines are notorious for some natural amount of oil consumption while driving, but a leaking oil pan will make this oil consumption seem worse. You should put a habit on inspecting the oil pan gasket for leaks every time doing an oil change. The chances are 100% likely that it needs replacing if you have never done it before.
I have noticed that simply changing the oil pan gasket without the proper procedure will make the oil gasket seem to leak after just a short while. This is usually due to lack of thread sealer on the pan bolts and improper torque. Sometimes the thin oil pan is bent slightly (due to bumping into rocks on a gravel road) and proper sealing is impossible to achieve, then you will need a new oil pan. Replacing the oil pan is something you might need to consider if you see clear scratches on the bottom side or noticeable dents.
The details on the specific oil change procedure for the M110 engine can be found in this article: How to Change Oil and Filter in the M110 Engine
You cannot do this procedure without doing an oil change, so try timing it to your next oil change otherwise you will need to replace that new oil of yours.
Parts and supplies:
- Oil pan gasket
- Thread sealer (not the same as thread locker)
- 6.5L oil + oil filter
- Brake cleaner
- Lots of shop paper/rags
- zip ties
- Jack + Jack stands or ramps
- Socket tools
- Hex bits
- Low range torque wrench
- Oil drip pan
Start with warming up the motor to operating temperature and raise the front of the car for better access. Drain the oil from the oil plug and let it drain until the dripping stops. Then you can start the process of removing the oil pan.
After the oil is drained you can start loosening the oil pan bolts with a hex socket. There is a lot of them so be patient. And not all of them are the same length! There are a couple of bolts with longer length at the right front corner which also holds a bracket to an oil cooler line.
Take care of holding the pan horizontal when you remove the last bolt, because there is still a lot of oil left in the bottom. Carefully lower it and then pour out the remains. There is most likely some sludge and debris left. Now the time has come to inspect the bottom of the oil pan. The leftovers in the bottom of the oil pan can tell the condition of the engine and if it has been properly maintained.
A common denominator is that silicone or RTV debris are to be found due over use in valve gasket sealing. I do not blame them since the M110 engine is notorious to leak after a few years, but still this debris can clog the oil pickup and damage the engine. Also the engines tend to leak more if they are not used on a regular basis, sounds funny right? But it is the sad truth. Long time storage of these engines without usage will dry out the seals and the engine will leak more oil.
My engine (the whole car) had been parked for 1,5 years and only started intermittent and not driven. So the oil in the lower oil pan was very sludgy and you can notice that moisture has started building up, which can be seen from the white color. This is not good for lubrication obviously, and was the reason I did this job in the first place.
Use break cleaner and clean the oil pan thoroughly so it is dust free inside. Also clean the outside so you can notice new leaks id they appear. If you have some small strong neodynium magnets you can attach them in the lower edge of the oil pan, otherwise the filter will catch most of the debris anyway.
After the oil pan is cleaned, there is another matter to tend to. Grab the oil pickup strainer and pull it off. It is probably very clogged from debris and gunk, so you better clean it now or never.
Whatever you do do not spray oil cleaner oil solvent into the lower engine since it can damage the rod bearings!!! Use only some paper or cloth and clean off the lower engine housing then pull off the strainer and use brake cleaner on your work bench.
After cleaning the strainer, put it back into the lower engine and start assembly of the parts! Put on the new flimsy gasket, since it will not stay n place use 3-5 zip-ties to secure it in place for assembly. After mounting the oil pan with some bolts on you can cut the zip ties.
Prepare the bolts with the thread sealer, start with two bolts, one on each side so the oil pan will stay in place.
Put thread sealer on each bolt yo inert, all the bolts should be tightened to 11 nm. Remember that the two front right bolts have a different light than the rest due to a bracket with and oil lines.
After torque of all the bolts this job is well done. Hopefully it will not leak in a few years, well done!