A couple of questions relating to pergola construction

[^woody^]

Just check the suitability of the product on horizontal surfaces. They may be good for vertical, but most state not to use on horizontal surfaces.

UV is the enemy, and invariably surface coating will flake and look patchy, whilst a penetrative oil will just fade and is easy to recoat. With stains, check if they are a true penetrative stain or merely a surface coat that changes the wood colour.

 

[mrrusty]

I struggled to find a true ‘oil’ product

https://www.birdbrand.co.uk/shop/woodcare/complete-wood-protector/ is not water based and wot I use.