You pretty much need an Adventure Pass or equivalent to park at most of SoCal’s trailheads except when you are on private land.
If you know you will be visiting National Parks and other fee entry based federal land areas over the next year, buying the Annual America The Beautiful pass for ~$80 is a better deal since it works for most federal land. I buy one every year and it works anywhere where an Adventure Pass is required.
As for parking at Devil’s Slide, I’ve never had any trouble with my car being left there for days at a time. Well, there was one time I got a ticket even though I had my Annual pass displayed because the ranger didn’t check very closely which pass I had and wrote I had the wrong one. I went to the office in Idyllwild and compalined and they dealt with it for me with no hassle. They mentioned that they’ve had other complaints about one particular guy who was half assing his job and that they’d talk to him again.
Miner