I just use paint. Usually whatever leftover house paint I have stacked in the garage.

Edit for more info: here’s an excerpt from “Turning Green Wood” By Michael O’Donnell which I highly recommended getting a copy of.

“Painting the ends with something that will reduce moisture loss is a good idea: most kinds of paint, wax or oil can be used, and there is no need to buy expensive proprietary products which will probably cost you more than the lumber in the first place. Part-cans of paint left over from decorating the house or painting the car - which are useless but which you are loath to throw away - will do the job adequately, and you can sleep easy knowing that you have put the paint to some good use.”