1994 3.1 lumina ac leak

[Hemi34]

I have a 94 lumina apv 3.1 v6 with single ac that has a freon leak, I cannot seem to find it, I have used a sensor and cannot detect any leaks on the hoses, the compressor has some oil on it but looks like engine oil,but does make some noise. I can charge it and it will last about 2 days, any suggestions where the leak can be?
Thanks David

[tomcat0813]

Check your shrader valves, look for dirty oil residue on your condenser, lastly check your o-ring in system