You don't say what year the vehicle is....
Having to add refrigerant every two years is a real small leak, and would not tend to lose too much oil.....
first thing in the morning, before the car is started, is when you should check the pressure....high and low side should be equal... and they should approximately equal ambient temp.....70 degrees is the magic number...at 70F, pressure will be equal to 70psi.....below 70F, the pressure will be slightly lower.....above 70f, the pressure will increase more.....for example, at 50F ambient, pressure in the system will be 45psi....but at 20 degrees higher, at 90F, the pressure is 104psi.....
Running pressures are a different story......low side should not be around 25 psi, because this can cause the evaporator to freeze....it can cause the compressor to shut off, or "cycle"......
these are typical numbers....at 70F ambient air, low will be 35-40psi and the hi will be 145-160psi.....If your pressures are lower, you need to add more refrigerant...