For Sierpinski k=18, I found 2 primes:
18*74^662+1
18*239^1990+1
Thus, there are only 3 bases remain for Sierpinski k=18: 18, 227, 293. b=18 was already tested to n=2^252 with no prime found, b=227 was already tested to n=1M with no prime found, for b=293, I found no prime, it is likely tested to at least n=5000.
