I would use none the timber is treated and will be ok for a few years I decked mine 5 years ago and it’s still fine.

I usually treat mine with one of the cheap garden wood products from a DIY superstore. That's mainly for cosmetic reasons though, and I agree with Steveg that if it was properly treated decking timber it should last for years.

You can't over protect so I'd use as bertie. Sais once a year. A lot of decking infact a lot of new wood isn't really Very good quality and quite green.