Have little oil pressure on 1981 landcruiser after doing oil change have checked pressure relief valve its ok, have checked crank bearing its ok what next

Assuming you are talking engines and not auto transmission, the oil pressure relief valve prevents excessively high oil pressure when a cold engine is started - cold oil is much thicker than hot oil and therefore doesn't flow as well as hot oil.

Some designs of pressure relief valve can sometimes stick open and this will become almost obvious as most of the oil pressure will be lost when the engine is warm, especially at idle when the oil pressure light will be on. In this condition the crankshaft and other bearing do not last very long...

For any engine with an oil pump, the components inside it are designed and engineered to operate at a certain range of oil pressures.
For example, an automobile engine might be designed to operate at # 30 to # 70 pounds of pressure.
If you subjected the bearings to double the pressure, you could achieve bearing failure, as you could with not enough pressure.
The oil pressure relief valve will "open" when the upper limit of pressure is reached and allow the extra pressure and oil to go straight back to the crankcase.

Has it any oil in it at all?

If the oil level was low heavy cornering and braking can cause oil swill away from the pump pick up robbing the engine of oil pressure for a few moments. At high engine revs it takes only a few seconds for permanent damage to be done.

Some vehicles use a steel oil pan that is quite exposed and if grounded the oil pan can be bent upwards and close off the oil pump inlet.

It isn't unknown for an oil pressure relief valve to stick open but that is unlikely remove all oil pressure from a healthy engine when cold.

Clearly there are some careful checks needed but if the engine sounds bad the damage was probably done Tuesday night so a crankshaft and bearing overhaul is probably needed...

The oil light will come on when ever pressure is low it ct can be as little as the oil pressure sensor is going out or you may have a blockage somewhere in the oil system try doing an oil change with mobile1 oil this may help if there is a small blockage but defiantly get the sensor checked first

check that the oil level is not over the mark as this causes foaming of the oil and loss of pressure. At 80km/hr you will be doing around 3 grand and that will be enough to whip the excess oil into froth.. Too much oil is worse than too little oil.

This could be worn bottom end bearings, a worn oil pump, a weakened oil pressure relief spring, or a faulty gauge. I once saw a Landcruiser with a faulty gauge.

Get another good gauge into the oil pressure port as a first step. If it gives the same low reading try a temporary fill of thicker engine oil. If this causes the reading to rise, it is probably worn bottom end bearings, not unusual in a 95 model.

After that you will need to inspect the oil pump.

The oil pressure relief valve should be on the oil pump.If your oil pressure is ok,have the wiring tested to theswitches or try another set of clocks

there should be an oil sending unit somewhere near the oil filter and if it is the sending unit this would have been alot cheaper then changing the oil pump

There is a pressure relief valve in the oil pump, it is stuck closed, this happens now and then, seen it 2 times, since the oil pump is a positive displacement pump it can produce 100's of pounds of pressure, the reason for the noise is that u have allot OF pressure but very little volume, the bearings are being starved for lubrication, replace the oil pump, DO NOT DRIVE THE TRUCK, U WILL BURN OUT THE ROD AND CRANK BEARINGS.