I have a cherokee jeep limited edition 1998 right hand drive

Does it drop to dangerous levels or just a little bit? Do you have high miles on your Jeep? Oil gets thin when hot and oil pressure can drop a bit normally when warm. What viscosity of oil do you run? Going up a step might help.

15w40, origanaly it was 15w 30 can it be the motor oil pump, and if so how can it be changed easily whitout lifting the motor

If the pan can be dropped with the engine in then yes. I am guessing a worn engine though. How many miles on your Jeep?

120,000 kilometers, yes it drops almost 70%. when the motor cools down it's normal pressure

i am worried that the motor can jammed due to no lubrication

That is a major drop and fairly low miles. Try a new oil pressure sender- no promises but it is worth trying. I would be worried too. Good luck.

so you rule out the motor oil pump? if after the replacement of the sensor the same problem stays, then i will have the pump replaced. thank you very much

I am not ruling anything out. I am just suggesting the easy stuff first. If it is a false low then great. If not then nothing really lost.

First; dont ask your buddie, mostly their wannabe idiots! Unless he/she is a grey bearded ASE MASTER. Oil Pres (OP) mostly is factory enginerring & function. 1) does it knock or tap loudly? 2) are u using correct fuel? 3) Smoking, leaking, vibrating, or any of the other obvious bad stuff?. If not, ur probably good for daily driver. Still concerned? Take the seller, buy him a 52 Oz paper soda with a damaged lid. Top off engine/AT bodily fluids, drink a beer, drive to safe straightaway, PUNCH it to the floorboard, wind it up to REDLINE through 3 gears, then deccellarate nice & easy to normal traffic speeds for a few miles. If it blew up, and ur still alive, you know your doubts were well founded. Don't buy it! If it runs smoothly & ur ambulatory & pants are dry; make a low offer, cuz your last one was faster. He or she, even better, will be embarassed, covered with red goo, so if they don't pull a gun, will be a lay down on negotiations. Take savings & treat your sweet machine to top grade FULL SYNTHETIC (NO BLENDRD CRAP) 10-40, K&N Oil & air filter, (good for 10k+ miles (BMW GOES 12.5 K miles) Bosch Wires, coolant flush (no BS caustic chemical) & stock temp thermostat, new front O2 sensors. FYI -Fords are engineered generally with higher (50-85 PSI) OP, GM favors 15-50, but Mopar (including Jeep), widely varies OP Depending on purpose. After proper oil temp is reached, idle OP drops on some of my Mopars to 15 psi, then right back to 70 psi revving to over 6k rpm (up to 10,000 rpm with some oil system mods.) Pt Master ASE