I'd be wary of filling the cracks, unless your seal is perfect (and movement in the wood will always be an issue) , water will get in, and then it might easily be trapped in place making things worse.
Personally I'd let it get nice and dry (spring maybe?) run a gaffer tape up the length of the crack, leaving the top open, then flood the crack with preservative, wait a few days and then do it again (and again until you are confident it has taken all it can), then remove the tape, and coat the rest of the post.
If you think the crack is likely to get worse, you could run a couple of heavy screws or bolt through side to side pinching the crack shut (or at least minimising further splitting
I think outdoor wood always has a finite lifetime, and for a heavy post like this I wouldn't be too worried