the Collatz conjecture, primes, and numbers that end in 6

All primes greater than 5 end in 1, 3, 7, or 9. Using the 3n + 1 rule of the Collatz conjecture, the corresponding even numbers end with 4, 0, 2, and 8 respectively. So there is no way an iteration of the conjecture on a prime number can produce an even number that ends in 6.

What happens when a number ending in 6 is used as a seed value for the conjecture? Here are some sequences:

6, 3, 10, 5, 16, 8, 4, 2, 1

16, 8, 4, 2, 1

26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1

36, 18, 9, 28, 14, 7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1

46, 23, 70, 35, 106, 53, 160, 80, 40, 20, 10, 5, 16, 8, 4, 2, 1

56, 28, 14, 7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1

66, 33, 100, 50, 25, 76, 38, 19, 58, 29, 88, 44, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1

76, 38, 19, 58, 29, 88, 44, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1

86, 43, 130, 65, 196, 98, 49, 148, 76, 38, 19, 58, 29, 88, 44, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1

96, 48, 24, 12, 6, 3, 10, 5, 16, 8, 4, 2 1

106, 53, 160, 80, 40, 20, 10, 5, 16, 8, 4, 2, 1

Some observations:

In 9 out of 10 cases in the above, the first odd number is prime. 

If the seed ends in 6 and is a multiple of 8, it converges quickly. 

Generally, as the seed value increases, it takes longer for the process to reach 1.

Further conjectures:

Large primes can be found by using the Collatz conjecture on large seeds which end in 6 and are not multiples of 8.