I can’t see any reason for using a fuel additive, particularly as you use high-octane fuel.

The best option for you is probably to ditch the run-flats and fit regular radials. You can then run a regular spare and you could buy a jack from your local auto accessory store.

YOU have one great car there. The 535i was the sporting flagship of the regular 5 Series until the V8s appeared in 1993. They are great to drive and pretty reliable. I would suspect your vibration is caused by one or more of the drive-shaft joints. Have it checked by a BMW specialist. As for fuel consumption, you do have a sporty car that is renowned for using more fuel than those with smaller engines. The fuel consumption you're getting is about right for the 535i. As for possible problems you may encounter, the 535i is prone to head-gasket problems at the sort of mileage yours has reached, and if not caught this can lead to more expensive damage to the cylinder head. They are also prone to radiator failure. These cost about $700 to replace and can lead to gasket failure if not fixed when you first notice a higher than normal temperature.