Air dryer purging oil?

Well here we go again... New air compressor (timed properly) with ISX CM871 engine, new air lines and air dryer and still losing oil out purge valve on dryer. This is 2nd compressor in less than 1 year and 1st one contaminated dryer with oil. Air restriction at charge air cooler? No oil loss at turbo and intake manifold is clean. The ISX compressor has no oil lines, just coolant. Oil is supplied to compressor from front gear cover. What am I missing here?

 

Thanks for the reply Heavyd. I'll pull off compressor and check oil drain for blockage. I still have original air cooler on truck. If there was a restriction through the cooler, would that cause my problem, if so, how do you check for intake restriction?