I'm also unsure about the oil to use.
I agree with comments about there being various good oils and running the mj on mobile 1 etc as long as it's the correct grade but.........
......as the 500 has a DPF and these can be damaged easily there has been a number of new standards set up for diesel engines specifically with DPF technology.
The reason I'm even getting involved in this thread is simply to highlight something I found out with another diesel vehicle I had when I needed to top up it's oil.
In my 'other car' the handbook stated I had to use oil with ACEA C3 or C4 rating (not 100% which). When I went out to find oil, loads had ACEA A and B ratings but not C, the requirement of C being tied to DPF type systems on the car.
The garage wasn't even sure and it turned out they couldn't even supply the oil to me in small 'top up' amounts as they had none. Eventually they did get the correct stuff. Even my local Renault garage whom I'm very friendly with didn't know anything about the ACEA C rating stuff.
This info gives some info on the standards
http://www.acea.be/images/uploads/pub/070308_ACEA_sequences_2007_LD_and_HD.pdf
Please note, I'm not trying to be a know it all and I'm sure someone on here will tell me I'm talking rubbish.
I looked in the 500 book and asked the dealer the day I collected mine for some top up oil and they said they'd just use the normal oil they use for all diesels (it's a Saab, Citroen, fiat dealer) which didn't impress me and as of yet I'm still unsure which northwest dealer to go to for aftermarket service.
Mine 500 is now at 5500 miles and ideally I'll have an oil change at 9000 if I find a good dealer and know they'll be using the correct oil.
Looking at Mobile 1 specs
http://www.mobil.co.uk/UK-English/LCW/Products_Services/Mobil_1_ESP.asp they specifically mention the DPF and EURO IV specs.