When cleaning the MAF (with crc maf cleaner - I personally would not use anything else), you are supposed to let it dry first (using the proper MAF cleaner it dries quite quickly) and then reinstall and test it.
Why would you think re-installing it "wet" would be fine?
What did you use as MAF cleaner (substitute)?

For a MAF cleaner substitute I used ... CRC MAF cleaner! I am just a part time, not a full time idiot! I know it has to be sensor safe and not a traditional carb or throttle body cleaner (unless it is specifically labelled as sensor safe, and even then I would avoid it). I know it doesn't have anything that will cause problems to the engine downstream, or you would never use it in the first place, because it is not like it completely disappears when it dries. Compressed air will dry it safely and more quickly, which is essentially what I did. Could it short and damage the sensor? Possibly but probably not since it is supposed to be less than 1v at idle. That is what I didn't fully consider.

Just checking when I ask... you'd be surprised what people will use to clean their MAFs, etc...
We're ALL P/T idiots (speaking for myself only....)
Have no idea if it can short out the sensors while still wet/damp with MAF cleaner on it. I've never re-installed a MAF NOT DRY and run the vehicle.
(on a side note: I have experienced an oil film coated MAF (c230K) and it ran like crap on acceleration until it was cleaned and the oil film issue on the intake side was resolved. I did not have to replace the oem maf)

Gettng back to your MAF replacement options.... oem vs. non-oem,
Again, just from personal experience (AND one only time....) AND from what vet members here have quoted on the use of non-OEM MAFs, I'd take the high road on these critical componmets knowing that there is SO MUCH generic/ counterfeit parts out there. :)

I thought this was kind of weird...

The MAF on the vehicle now came with it from the original owners at 162K. I remembered it had a fresh looking bar code, so it definitely was not 20 years old. Well, it is a Bosch, but it goes downhill from there (I think?).. First, on the barcode sticker it says it is remanufactured. Second, as you can see in the photo below, it is made in Korea and is marked for Hyundai and Kia.

Question: Is this supposed to be a compatible part, and if so is a Bosch MAF for a Kia the same as a Bosch MAF for a Mercedes?

https://cimg8.ibsrv.net/gimg/www.mbw...6ab6ff27ad.jpg

That part# looks suspect. I dont think it's the correct one.
oem Part # for a 99 ML320 - # 1120940048

It is definitely suspect, but it is also definitely a legitimate Bosch part number for a Kia or Hyundai.

It is also definitely a direct fit for the ML320.

Weird all around.

Not so weird. You'll find many vehicles components and parts are interchangeable (with diifferent part #s). I found out recently reading thru/ sifting thru alot of peripheral threads that some MB class vehicles share parts with Chrysler (Cross-Fire as an example), and also amongst the different classes (Our ML500 and C230 share the same motor air filter). There's many more. The MAF may fit (size wise) but the sensors may be different.

My understanding is the Crossfire was built when Mercedes owned Chrysler and it has the same W163 engine as the ML320 so most part numbers are the same. The MAF for a Crossfire is literally the same part as for the ML320.

Rather than two related cars sharing the same part (Crossfire and ML320), this seems like two unrelated cars that for whatever reason are using the same form factor for the MAF but with their own part numbers. I haven't pulled the sensor out yet (I don't have the right size security torx). I'll be very curious to see what is the part number of the sensor itself, since the bodies holding them look identical.

It still have the question of how many cars share that form factor and whether the sensors that fit in them are compatible.

Haven't really looked into the details of the shared parts; only thru reading and some curiosity, and where it may affect our rides for le$$ expen$ive replacements. Some of the members here have a good respectable knowledge of cross-references for their own mb rides and have shared what they know thru postings. From what's been posted, their is obviously good savings for both new and used parts when it comes to compatible components (albeit difference part #s)
It would be good to have a compiled list of compatibles (a dream come true for alot of the DIY members here and abroad) :)

I bought an aftermarket MAF as a test, and it made a substantial difference. For one bright shining moment I was really happy. No noticeable oil leaks, and the engine was running smoother with increased power. The idle seemed just a little rough, but not 100% sure whether that was just my imagination.

With it's age I usually drive it very gently. I accelerated hard to see what it would do, and at about 40MPH what it did was throw and shred the belt. Aaaargh!

I am going to create a new thread for that, but I wanted to close the loop here on the oil leak and the MAF because both of those look very good at this point.

Thanks for the update on your issue.
You may also want to check your vacuum lines for cracks/leakage and replace as needed.
GL.