I haven’t tried going really low with my ‘18. I run BFG All Terrain T/A KO2 in load range E, which run 45 psi on the road. I’ve lowered them only to 35 to date, mainly just to keep the TPMS sensors from turning on the dash light.
Before my 4Runner, I had a ‘02 Tacoma TRD Off Road. I also ran BFG All Terrain T/As on there (the KO2 wasn’t out yet). I ran stock size tires on OEM rims, 35 psi street pressure.
Back then I lived in eastern California and the eastern Sierra Nevada, Death Valley and central and southwestern Nevada was my play ground. I generally aired down to 20 psi in the front, 18 in the rear. Often, instead of waiting 45 minutes for a red hot 12v plug in air compressor to bring my tires up to street pressure, I’d just drive home at trail pressure. Even in summer, the tires often weren’t too hot to touch, but the truck was noticeably more squirrely. At home I’d air up with my shop compressor.
I’ve often heard that one wouldn’t want to attempt to go below around 15 psi before going with bead locks. That’s for Wranglers and other light vehicles. Our T4Rs are considerably heavier.
I’ve found that even a drop of 10 psi works well for most situations.