Diesel oil and tar are non-toxic? Maybe compared to the arsenate-treated and worse, creosote-treated stuff. An oil-tar mix would be a good herbicide, similar in action to the old "carrot oil" used to kill roadside and railbed weeds, including some hard-to-kill perennials.

Cedar is naturally rot-resistant but weathers irregularly. Most treatments will darken the older wood. If you like the color already you might try a water-proofing treatment applied several times over the course of a week or so. You can also use boiled linseed oil thinned with perhaps paint thinner (is there a less toxic alternative?) and brushed or sprayed liberally. The surface will darken.

If you ever want to darken/"age" new redwood and some cedarwood, make a "paint" of baking soda and water and brush on for a darkened look. I do that on redwood benches and fencing for that instant weathered look. The dark colors fade with time to true aged ones.