What to use for outdoor anti-rot/mildew/water wood protection

[Goedjn]

Paint.

[blueman]

I know that for posts and other wood protection they sell some nasty compounds with multiple warnings.

What about for wood that is outside and exposed (like clapboard or fence rails) but that is not touching the ground?

I went to my local paint store and they said just use a good oil-based sealer. What do you folks recommend?