Hi @snipwhip,
As timber is a natural material with grain lines that run throughout, cracking like this along a grain line is pretty common and natural weaknesses can be exacerbated when pressure is applied to the timber when it is screwed in place.
As the cracks are on the upper face of the boards, they do pose an issue as they can become pooling points for water, which can lead to the premature rot of the timber. They do not pose an imminent issue, but over time, water could sit in these cracks and find its way inside the timber regardless of whether it is oiled or not.
For this reason, I would consider replacing them or rotating them so the crack is facing down. Assuming the crack is only on the upper face, this would prevent water from pooling in the cracks, which will prevent the issues I mentioned above.
Let me know what you think and if you have further questions, please don't hesitate to ask.
Jacob
