Greasy cv boot less than 2 yrs
 

I got both cv boots replaced less than 2 yrs ago but one of them is greasy now. The car is only driven on urban road, no gravel road at all, no sharp turn
, and never turn steering wheel when the car is stand still.
How could it broke prematurely? Could it be improper installation? Could it be cheap part?

It could be cheap part if you didn't install OE BMW or GKN Loebro (oe boot is made by them)

Those have a ridiculously high failure rate even OE. I've replaced them on a 30,000 mile 3.0i and a 25,000 mile 4.6is.

A case could be made for age, but I've had older cars with higher mileage that had longer lasting boots.

I just count them as more "character" - building quirks of E53 ownership...and I sure have built up a shit-ton of character!