Have your vehicle on the bitumen and let your tyres down to 18psi and note the sidewall bulge then let them down to 15psi and note the sidewall bulge.If there is bugger all difference it won't make much difference on the beach except to break the tyre off the bead easier.Do this at home to one tyre and see the difference.Had 3 hilux's a 60 series and now a Prado all manner of tyres 18psi when on the beach never had any problems,you need momentum when getting up the soft tracks and choose your gear wisely.For instance stop vehicle on the hard sand before the track you wish to get up put it in low range in 2nd gear take off and then 3rd gear and give it some stick but don't go beserk.When you feel that the vehicle is going to make it back off some and steady as she goes.You will get feel for what your vehicle can do after a while and what tyre pressure your comfortable with.Gear selection is very important when climbing sand and power doesn't go astray either.Just be careful when deflating to lower pressures as breaking a tyre off the rim needs special tools to put back on.Jim