What should oil pressure be on a 2005 mustang 4.0 hot at 2,000 rpm? Cold, 75 degrees it shows 50 psi.at 800 rpm. When at normal temp it shows 15,20 psi at 800 rpm.

That sounds normal. There would be specifications for it that you should check, but that sounds pretty normal. The specs probably show a pressure for idle at normal operating temperature, and another figure at a specified engine rpm. Or it may just have one spec at a certain rpm.

There should not be any pressure.

What engine ? 4.3 , 4.8 , 5.3 , or 6.0 L
Oil Pressure - Minimum - at Normal Operating Temperature
42 kPa at 1,000 RPM
125 kPa at 2,000 RPM
166 kPa at 4,000 RPM
6 psig at 1,000 RPM
18 psig at 2,000 RPM
24 psig at 4,000 RPM

4psi idle (bad) and revving to 3k does what, you didnt say.
what was idle RPM 800 or 400>?
the answer is different by hot and cold oil
and oil type, no type stated (do not say brands) 10w30?
running 0w are you>?
and is as low as 8psi hot. at idle ,that is if idle speed is at spec.
is it?
and 3000 rpm would be like 25psi hot, and more cold.
the max pressure will be the pump regulator value.

here is the sensor spec.

The sensor closes when the oil pressure drops to between 4-7 psi (28-48 kPa).

the oil type and bearings clearances (in total) dictate pressure.

3,1 CPC engine spec is.

55 is max (pressure relief)
20 is typical hot engine at 800 idle. and rises as RPM rises.
your post tells neither , temp, or RPM

somewhere around 1.6 pounds (1lb.14ounces) of R134a if system is evacuated and 3 ounces of oil with new compressor.
keep pressure on the low side around 34-37psi & it should blow snow. Below are normal car AC pressure readings with 134A.

• Normal readings on high and low side with AC OFF (static pressure) – Depends on outside temperature, but normally is between 80-105 PSI
• Normal low side reading with AC on high speed and MAX & engine at 800-1000 RPM’s – Ranges from 25-35 PSI –
• Normal high side reading ranges from 200-350 PSI

Don’t assume that if adding little Freon is good that adding a lot is better! Overcharging just a little can decrease the performance of the system and possibly damage the compressor.
DIY recharge kit:
With the AC on the coldest setting, use a thermometer in a middle vent. Normal vent temperature readings will vary depending on the (ambient) outside temp. The vent temperature should range from around 42-55 degrees in my experience. If normal gauge readings are obtained and the vent air is cold – STOP don’t overcharge the system.
Thank you for using fixya and be safe.

Without knowing where you are or the temp/humidity conditions it's a tough call.You should understand this:too much freon is just as bad as not enough when it comes to A/C performance.Have someone help with this.Run the engine at about 2,000 rpm for 4-5 minuets,A/C on,fan on high speed.On a 70-75 deg.F day with moderate humidity, I would like to see 30-35 psi low side,225-250 high side.(even 275 isn't too bad).The hotter/more humid the day,the higher both pressures will be.Holding the engine rpm while testing is very important!If you see over 40 psi low side you could be over charged.Make sure the radiator fan(s)are working properly.

at what? depending on engine temp and rpm, normal oil psi should read 30-40 psi at idle and at cruising speed of 1500 to 2000 rpm should read 60-70 psi. most bonnevilles from 1992 to the last year run of 2005 have the 3.8 liter V6 which was updated over the years.

Minimum is usaully 10-13 lbs at idle. A good rule of thumb is 10 lbs per 1,000 rpms.

30 PSI hot idle is ok, 40-50 PSI at freeway speeds hot is good. this is based on using the correct weight of oil which is 10W-30W