I have a post for a pavilion that has started to crack. The cedar exterior of the post must be a facade because by itself it was like 20-30 lbs - much lighter than expected for a 7' 6x6 post. This specific post holds weight but maybe only 50 lbs worth. Also I'm not sure how much of a difference it makes but we are in a desert near the Rocky Mountains.
Would I be fine with regular wood putty or do I need something more substantial?
That's not a crack, strictly speaking. It's "checking", and it's what happens when a wooden post shrinks. It's virtually unavoidable, and it's not a structural problem. Have a look at almost any other similar post, cabin beam, etc. Every single one will have at least one split like this parallel with the grain.
Attempts to fill it will not only be futile, but it'll look like poop. Literally and figuratively. Wood expands and contracts across the grain, and it will continue to move seasonally. Any filler or patch that isn't highly elastic will still show a crack. It will also match the color and texture of the wood poorly.
I encourage you to embrace the nature of wood and not attempt to make it look or behave like plastic.
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