I recently checked out one of my thermo king reefers . the unit is sb-310 . Unit was running and I didn't see any oil in a sight glass in compressor. I heated it up for 30 min and after put on cool high speed and still nothing. When i shut down unit and restart and unit was running at a low rpm on cool I noticed there is some drops of oil hitting the sight glass and the oil was barely seen in the bottom of glass. After the unit switch back to high speed no oil was visible . Should I add oil to it?