Wet with oil, you mean engine oil? I've had this happen twice. Once, on a BX, it was an air leak. On a petrol car, it was an intermittently blocked oil breather pipe. So, check for air leaks in the diesel line, they must be pre diesel pump if there are no diesel leaks, as the air is being sucked in; and for any kinks or other problems in the oil breathers which are forcing oil vapour up into the inlet. A lot of oil breathers go the other side of the air filter so as not to blacken it too quickly.