There are these:
BMW - 11 42 7 673 541
Walmart - ST3614
Fram PH6063
Purolator ML16825
Bosch - 3330
Advance AutoParts AA3614
All of these are good, adequate filters (even the Walmart).
I have used the BMW and Bosch on my RT; the Bosch run about$6 - $7 at Autozone. I'm going to assume that the K1600 still uses the common filter.
The CalSci site is a good recommendation.
Please note that they are all about an inch and a quarter longer than the BMW filter. To date, I have not found an 'exact' aftermarket replacement filter.
Now, all that said, my dealer charges $17 for the BMW filter, which is only about 28 cents a mile vs 12 cents a mile for the BMW filter

. You might consider running the BMW filter for the first few changes. There shouldn't be any warranty issue, but...
Tom