Couple of questions
 

I recently bought a CPO 2012 X5d, with about 30k miles. This is my first diesel.

First question: Is there any fuel additive that actually works, e.g. prevent carbon build up etc (or just more snake oil)??

2. I have new Run Flats, but like my last 2 vehicle, I will go conventional tires when these wear out. I have a product in my Harley, call "Run-On", that's a puncture prevention and balancer. Has any one used this in the 4 wheel vehicle ??

Thanks

1) Carbon build-up where?

2) Better check to make sure that stuff is compatible with the TPS. I would think it would cause them not to function. What you probably want is something like this:
Penray introduces emergency tire inflator | Truck Parts & Service | Trucking Aftermarket | Heavy-Duty Truck Parts and Services

my mistake, it was Ride-on, and they claim its TPS compatible...

Tire Sealant | Tire Protection System | Ride-On - Ride-On

TPS isn't inside the tyre on an x5 e70

Any particular runflats? or just runflats?

Tires fail for reasons other than a simple puncture. Get a jack and spare...turns a 3-4 hour total waste of your life into a 10 minute task.

It is on the US models, there is a sensor in each wheel. On the Canadian models, it just uses the ABS sensor to detect different rotation speeds.
I have an inflator, plus dynaplug. I also have the BMW mobility system with sealant (which would be a last resort)

Sorry my mistake. In the UK they use the ABS sensor to compare wheel speeds, I wrongly assumed it would be the same the world over!

BMW uses two types of systems TPMS and FTM (I think). TPMS uses pressure sensors in the wheel and FTM use the wheel speed sensors to compare rotational speed against the other tires (a tire low on air pressure gets smaller and thus rotates faster as compared to the others). My 1998 E38 (system retrofitted by me) and 2004 E60 uses the FTM system. My 2008 E70 uses the TPMS. I believe in the USA, only the TPMS systems is allowed by DOT regulations at this time. Not sure when the change occurred. For sure, the FTM is the more simple solution, but probably not as sensitive to pressure changes.

Not a fan of any tire goop permanently in a tire other than maybe an accessory can for an application in a "I can't fix it" emergency...

150,000+ miles on my m'cycles and have had one flat, I think, long time ago.

Cars: a few flats in 53 yrs, but always discovered in driveway.

If the OP doesn't want to chase the 'spare/jack/tools add on', a decent puncture kit, a couple of tools and a plug in compressor offers a decent safety net, in my experience.
GL, mD