FR 525 has always seemed a bit rough, but as recently as last month a passenger car could drive to the Loy Canyon Trailhead, but no mas!
I went out there today and the road had washed out in a couple of places, presumably from some torrential rainstorms. One place in particular would be unpassable for a passenger car. You don't need a 4WD, but you do need a high clearance vehicle, preferably one with some all-terrain tires. My 2WD F-150 made it, but I wouldn't be overly excited to drive the road again. Hopefully they will re-grade it at some point.