cat challenger 65c last year with mechanical 3306??

i changed the oil this spring and it has been sitting for a few months i noticed that the hydraulic tank is getting lower and the engine oil is getting higher. where in the system would these two come together that it could get in the engine oil. is there a hydraulic pump somewhere that is leaking thru or a cooler? this is the last year of the 3306 after that they where 3176 i think