2010 Jetta TDI, the AC compressor load is 10 nm at idle

I have noticed my fuel economy suffers more than it used to be when the AC is on. Checking with VCDS I noticed it is requiring 10 nm of force at idle, and when I rev the engine to 2000 or more, the compressor load is never less than 3.5 nm. I had also checked my other Jetta TDI and it seems the compressor load is about 1.5 nm at 2000 RPM which is far less.I had changed the AC compressor control valve some time ago and AC seems to be cooling OK. But the high pressure gauge is also low (150-175 psi while my other Jetta is at 250 psi at the same ambient temp) while the low pressure gauge reads normal.I had added some oil during the charging of the system, would that be the cause? (Frankly I did not measure the amount of oil, but I am doubt I put excessive oil..) The car has 256K mileage.
Or, is my AC compressor on its last leg? Thank you for the information..