Probably the artist oils didn't stick as well as a gel stain. You didn't mention putting a clear finish over the artist oils. If no clear was used then this might be the problem.
All stains, gel or not, need protecting with a good clear finish. Generally, quick fading and possible peeling takes place if not protected with a clear finish. Any good clear finish can be used as long as it is rated for exterior use. Example; exterior spar varnish, polyurethane or acrylic urethane.